Perhaps I just have a bias against parallel dimensions (as evidenced from my interpretation of Silent Hill's otherworld), but I want to take some time to clear up what might be a mis-conception in the conventional wisdom interpretation behind Dark Souls' multiplayer summoning mechanics. Dark Souls co-op is not necessarily based on parallel dimensions, as many players seem to assume. It might, in fact, be intended to be an abstraction of time travel. I've noticed that many players online already seem to refer to the multiplayer mechanic of these games in terms of time travel, but I've yet to see any wikis, lore videos, or blogs that seem to explain multiplayer as a time travel mechanic.
I want to preface this analysis by stating that the below interpretation is not necessarily the absolutely, 100% correct interpretation. Individual players may disagree based on their own reading of the game, and I'm personally somewhat conflicted on the topic myself. I merely want to propose this as a possible alternative to the defacto "parallel worlds" interpretation. I'm going to point out in-game evidence that supports the idea that Dark Souls' multiplayer is based on time travel, but there is also in-game evidence and mechanical evidence that also contradicts that interpretation. I will address those contradictions as well. So that being said, please keep an open mind, and enjoy the read!
The summoning mechanic
So there are two games in the series that are not part of the Dark Souls franchise, and which have different in-game explanations and rules for the same multiplayer features (more or less). Those games are, of course, Demon's Souls and Bloodborne. Both have asynchronous multiplayer and summoning mechanics that work similarly to Dark Souls.
Demon's Souls summoning operates under the idea of summoning the spirit of a fellow adventurer who's soul is trapped in the Nexus. This is why you must be in soul form in order to be summoned. Bloodborne's beckoning operates [similarly] under the principle of manifesting hunters out of dreams (and I'm still trying to piece together the mechanics of it all). In Dark Souls, you aren't necessarily summoning ghosts (as you do in Demon's Souls), since the undead in Dark Souls are more akin to zombies than ghosts. Also, characters in Dark Souls can leave summon signs whether they are hollowed (dead) or in human form (revived), which is a significant alteration from Demon's Souls. A lore reason for summoning is provided in Dark Souls:
Solaire explains to us how summoning works:
"We are amidst strange beings, in a strange land.
The flow of time itself is convoluted; with heroes centuries old phasing in and out.
The very fabric wavers, and relations shift and obscure.
There's no telling how much longer your world and mine will remain in contact.
But, use this, to summon one another as spirits, cross the gaps between worlds, and engage in jolly co-operation!"
Both Solaire's dialogue, and the White Sign Soapstone (along with other online play items) make references to other "worlds", which leads to many jumping to the conclusion that each player's game is a sort of parallel universe within the Dark Souls lore. However, this may not necessarily be correct. Both Solaire's dialogue and the soapstone also provide explanations for these worlds: "time is convoluted | distorted". This seems to be the explanation for what is meant by "worlds", and it seems that Solaire and the in-game descriptions may be using "time" and "world" interchangeably. The phrasing in the white soapstone's description joins "the flow of time is distorted", and "the White Soapstone allows undead to assist one another", into a single, compound sentence, which definitely implies that the two phrases are linked.
"Online play item. Leave summon sign.
Be summoned to another world as a phantom through your sign, and defeat the area boss to acquire humanity.
In Lordran, the flow of time is distorted, and the White Sign Soapstone allows Undead to assist one another"
The dialogue of Saulden (the Crestfallen Warrior of Dark Souls II) is even more explicit. He uses similar language as Solaire, but instead of saying to "summon one another as spirits, cross the gaps between worlds, and engage in jolly co-operation", Saulden specifically states that by using the summon signs on the ground, you can "call upon spirits from other worlds through the schisms in time." He basically gives the same explanation, but replaces the phrase "gaps between worlds" with "schisms in time".
As far as I know, there is no other reference to multiple "worlds" within the games. Nobody ever talks about there being multiple versions of any given NPCs, or parallel versions of any areas of the game that exist in different states simultaneously, with one possible exception (and we'll get to that later). However, all the games do make references to "ages" and "cycles". There is the Age of Ancients, the Age of Fire / Age of Lords, and the Age of Dark / Age of Men. The game also makes frequent mention of undead, heroes, warriors, champions, etc. that have all come before you and have failed at their task. So in any conversation outside of multiplayer feature tutorials and item descriptions, there is absolutely no reference to parallel dimensions or other worlds.
Crestfallen Saulden specifically states that other players are summoned through "schisms in time".
Heroes, centuries old, phase in and out
If true, this would mean that when you summon another player (or an NPC, for that matter), you are not necessarily summoning someone from another universe. Instead, you are summoning a character from another age. The same is true for you. When you lay down your summon sign, you are not necessarily meant to be entering another universe; rather, you are being transported in time to the other player's age.
Aside from maybe Solaire and Lautrec (who do refer to other "worlds"), none of the other NPCs are portrayed as coming from another dimension or world. Instead, they are exploring your world with you. Seigmeyer, Logan, Lucatiel, Hawkwood, and so on all clearly exist in your world with you. They face the same trials and tribulations, and changes that you enact in your world also affect them (and vice versa). In fact, in the first Dark Souls, these character who you interact with (aside from Solaire and Lautrec) are not summonable. The NPCs that are summonable (such as Tarkus, Leeroy, and Beatrice) are all long dead heroes of ages past. We find their corpses and gear in the game world - in our world.
The games make frequent references to ages, cycles, and the undead who have come before you.
Dark Souls II doesn't exactly follow this precedent though (and wasn't directed by Hidetaki Miyazaki). NPCs like Lucatiel, Pate, Benhart, and Manscorpion Tark all appear as in-game NPCs that the player can talk to, and they all appear as summonable phantoms to help with boss fights. So are these Dark Souls II characters representative of the rule, or exceptions to it? Remember, that Crestfallen Saulden in DSII specifically referred to summon signs calling spirits "through the schisms in time". So Dark Souls II may be breaking its own rules. In any case, none of these characters talk about being from another world, as Solaire did, so they definitely don't come from parallel dimensions either.
Most summonable NPCs in Dark Souls I and III are already dead.
Dark Souls III (once again under direction from Hidetaka Miyazaki) follows a similar precedent to the first game: most of the summonable NPCs are characters that are already dead. One NPC that I know of who also shows up in-game as a living NPC is the Sword Master who shows up outside of Firelink and drops the katana if defeated. However, he can only be summoned if the player has already beaten him. The only other exception is Sirris of the Sunless Realms. She can be summoned as a phantom for a number of bosses, but only after progressing to a certain point in her questline. You can also be summoned into her world to help her defeat other NPCs. Upon completing her quest and helping her kill her grandfather (Holy Knight Hodrick), her corpse will be found at Hodrick's grave.
Correction: The above list was compiled from an incomplete list of summonable NPCs compiled early after the game's release. I have since found that many living NPCs in the game are, in fact, summonable in Dark Souls III, including both Anri and Horace, Yuria, and Hawkwood. As far as I know, none of them make mention of a parallel world, with one exception.
Dark Souls also included Lautrec, who appears as an NPC in-game, can be summoned as a phantom for a couple boss fights, and who has a questline that involves the player using the Black Eye Orb to invade him and kill him to retrieve the Firelink Firekeeper's soul. After we kill Lautrec in this invasion, his corpse appears in the Anor Londo cathedral, and we can loot his body to get his armor. However, if we kill him after freeing him from his jail cell earlier in the game, he only drops his Ring of Favor and Protection, and not his armor. Could this mean something? We'll return to this idea in a moment...
The Black Eye Orb invasions in Dark Souls I and III may involve killing these characters in the past.
There is also a Black Eye Orb in Dark Souls III. This one allows the player to invade and kill Ringfinger Leonhard after he murders Rosaria. Like the Lautrec NPC invasion, this one also takes you to Anor Londo.
So what do Solaire, Lautrec, Sirris, and Leonhard all have in common? Well, for one thing, they all act as sort-of tutorials for the games' multiplayer mechanics. Solaire teaches Dark Souls players how to summon help. Lautrec teaches those players how to invade. Leonhard teaches Dark Souls III players how to invade. And Sirris teaches those players how to use the Darkmoon covenant works (i.e. punishing the guilty). All these characters may be long-dead heroes that are simply phased into your time. Some are there to act as allies; others are there to pillage. Sirris, specifically, appears to be a Blade of the Darkmoon, but Yorshka says that "long ago, our company lost its last proper knight." Could she be referring to Sirris? And if so, does this mean that Sirris is long-since dead, or simply that she abandoned her duty?
Perhaps these characters are demonstrating a sort of predestination paradox, in which the player character is pre-destined to kill them. After all, after defeating Lautrec, his body (long-decayed) just appears in Anor Londo upon returning from the invasion. Sirris' corpse appears in the cemetery, along with Hodrick's grave. Leonhard's gear appears in the Shrine handmaiden's shop. It's as if they'd all been dead all along, but we couldn't see it unless we completed their quests. If the explanation is that we are travelling to a parallel world to kill these NPCs, then it wouldn't make sense for these corpses and gear to just appear in our world like that. If there are parallel worlds, then the corpses and gear should remain in the NPCs' worlds, shouldn't they?
Explicit time travel in all three games
Perhaps more concrete evidence is that all three games do provide explicit examples of time travel. The Artorias of the Abyss DLC involves literal time travel, as the player travels to Oolacile before its destruction. Furthermore, the character of Marvelous Chester claims to be from the distant future (and strongly resembles a hunter from Bloodborne - coincidence?). Most importantly though, is that we aren't there to simply observe. We actively participate in the shaping of lore that's already been explained to you in-game. It's a sort of grandfather paradox or a predestination paradox. I say "sort of" because the DLC is optional content. Since the player may never actually play the DLC (or finish it to completion), then the events must have happened regardless. So I guess if you don't kill Artorias, he perhaps goes back and finishes the job of killing Manus. Or maybe Ciaran does it - or heck, maybe Alvina does it! Or it's a contradiction.
The Artorias of the Abyss DLC provides us with an explicit example of time travel.
In Dark Souls II, we also participate in the past via the "memories of the giants" at the end of the game. This is a bit more observational, as we don't seem to directly change the past the way we did with Artorias. Instead, we seem to simply participate in events that would have happened anyway. It's also possible that the DLC stages of Dark Souls II may also involve time travel.
Dark Souls III may also have some fairly explicit time travel in the form of the Untended Graves and Dark Firelink areas. If the player defeats Consumed King Oceiros, you can travel through an illusory wall and step out into a version of the Ash Cemetery and Firelink Shrine that is cloaked in darkness. In this area, you'll find only one living NPC: the shrine handmaid. If you talk to her and then return to the regular Firelink, she'll ask "Haven't I met you before?", implying that your meeting with her in the dark shrine took place in the past - a past that probably takes place during one of the Ages of Dark in the cycle.
Both Dark Souls II and Dark Souls III offer examples of time travel
via the memories of the giants and the Untended Graves.
Ash Cemetary and Untended Graves are actually the same space in the game. Any messages or bloodstains that appear in Untended Graves or Ash Cemetary also show up in the other, and the save file actually indicates that they are the same place.
The only other character that we encounter here (thought there are a number of enemies as well) is Champion Gundyr - a Gundyr who is not afflicted with the Pus of Man and who still possesses his soul (as evidenced by the fact that we acquire his soul after defeating him). His soul can be used to transpose Gundyr's Halberd, which "is said to never crumble, seeming to suggest that Gundyr was fated to eternal service from the beginning." This implies that Gundyr's fruitless quest may have been a sort of predestination paradox designed solely to preserve the Coiled Sword long enough to test a future Champion of Ash - that champion being you.
Space is convoluted in Dark Souls III
Dark Souls III then further obfuscates the world of Dark Souls by adding convoluted space into the picture:
"Yes, indeed, it is called Lothric,
Where the transitory lands of the Lords of Cinder converge.
In venturing north, pilgrims discover the truth of the old words.
The fire fades and the lords go without thrones.
When the link of fire is threatened, the bell tolls,
Unearthing the old Lords of Cinder from their graves:"
Miyazaki could easily have set up each Lord of Cinders in Dark Souls III as being from his own "world", which would have cemented the interpretation of each player playing the game in their own parallel dimension. He chose not to do that, and instead made each Lord of Cinder be an historical character who had successfully linked the flame in an age past. This maintains a singular world with a singular history (unless you decide to interpret Dragonslayer Ornstein as proof of parallel timelines).
In fact, the "converging space" of Dark Souls III could even be applied retroactively as an explanation for the impossible geometry of the world of Dark Souls II. If both space and time are literally in flux, then the incoherence of Dark Souls II's map design makes sense. Though it's still worth noting that Dark Souls III managed to pull this off without needing impossible geometry...
So what's the deal with Solaire?
Solaire remains an interesting question (and problem) in Dark Souls lore. If he does indeed hail from a different time (or even a different age), then why does he phase physically in and out of your world, while other heroes (except maybe Lautrec) don't? I don't really have a good answer to this question.
If Solaire hails from the past or future, then is he at the Kiln to defeat Gwyn? Or to defeat you?.
The idea of Solaire being a time-traveler does bring up an interesting question: is he from the future, or the past? If Solaire's summon sign is sitting outside of the Kiln of the First Flame, then is he there to kill Gwyn? Or is he there to kill you? After all, he does say that your fates appear to be intertwined, and it's unclear whether his intentions are entirely noble.
Contradicting time travel as the summoning mechanic
As mentioned at the top of this analysis, this time travel interpretation is not necessarily correct. In fact, there are also many pieces of evidence within the game (including the way that the game mechanic itself actually works) that contradict the possibility of time travel being the foundation of the multiplayer mechanic. I'm going to go through some of the major contradictions (and try to counter them, if possible), starting with the most obvious one:
The actual mechanic is literally parallel worlds
The actual implementation of the multiplayer mechanic is, by definition, a parallel worlds implementation. I play the game on my console or PC, and each of you play the same game on your consoles or PCs. We play through the same events, fight the same bosses, interact with the same NPCs, go to the same places, and so on. And all this happens to each of us at the same point of time in the game's history.
That sort of design is, however, a practical necessity. In virtually any game, elements of story-telling have to be sacrificed for the sake of mechanical playability. The player character in Dark Souls, for example, does not truly go hollow the way that other characters do. While NPCs die as undeads, go hollow, and then can be permanently killed by the player, the player is free to die indefinitely and always comes back to fight again. So even though the idea of coming back from death is written into the game's story (rather than just restarting us from a save point or checkpoint and "retconning" the story the way that other games usually handle death), the player's death is still a special exception that is treated differently than NPC deaths.
Hawkwood explains that undead and unkindled "can't even die right".
So instead, we have to think of death, hollowing, and restarting at a bonfire to be abstractions for how the undead curse actually works, because it seems to work differently for the player than it does for NPCs. The multiplayer mechanic being an abstraction for time travel may be the same way.
NPCs skip over bosses and obstacles
Perhaps an even bigger counter to the possibility of time travel is the fact that even in-game NPCs seem to blatantly violate the possibility of time travel. After all, if all the NPCs (and other players) existed in the past, then how did any of them make it past the Taurus Demon (the first proper boss in the game)? The Taurus Demon is, after all, still alive when I play!
For a more specific example of what I'm talking about, let us (once again) look at the story of Black Iron Tarkus. If Tarkus ascended Sen's Fortress, reached Anor Londo, broke the window into the Painting Guardians' room, and then died at the hands of the guardians, then surely he must've killed the Iron Golem. And if he killed the Iron Golem, then why is the Iron Golem still guarding the gate to Anor Londo?
If Black Iron Tarkus ascended to Anor Londo, then surely he must've killed the Iron Golem, right?
Well, there's a couple explanations for that. The cyclical ages may be one explanation. Perhaps the given boss or obstacle simply wasn't present in the past age when the summoned phantom existed. Or perhaps the bosses are subject to the same cycle of undeath and rebirth that the player is. If the player character and many NPCs are undeads who "can't even die right", then perhaps the bosses are too. Maybe they also eventually resurrect? After all, the standard enemies mostly resurrect whenever the player dies or rests at a bonfire.
Anri and Aldrich provide an irreconcilable hurdle
One very specific instance of the above paradox involves the questline of Anri of Astora in Dark Souls III. Anri and Horace are searching for Aldrich in order to seek revenge on him for trauma that they endured at Aldrich's hand as children. If you progress Anri's questline to a certain point, you'll be able to activate her summon sign outside of Aldrich's chamber. Instead of summoning her to help you, you'll instead be warped to her world where you can fight Aldrich. Killing Aldrich while summoned earns you Anri's gratitude and a gift of her sword, but it does not kill Aldrich in your own world! You must still fight Aldrich on your own in order to acquire his Lord Soul!
Anri can summon you into her world to fight Aldrich.
What makes this particularly confusing is that there is already another example within Dark Souls III of being able to assist an NPC character with completing his questline and defeating a boss. That character is Seigward, and he appears in the Yhorm boss encounter at the completion of his quest line. He simply steps into the boss arena during a cutscene to join you in battle. No summoning (of you or him) required.
Anri's battle with Aldrich isn't an abstract representation of you fulfilling another player's storyline in their version of the game. This is the one (and only - that I'm aware of) concrete example of multiple versions of the same boss being killed within single-player continuity. This is almost certainly the biggest hurdle to the time-travel hypothesis, and almost single-handedly breaks the entire idea. Quite honestly, I don't really have any compelling way to counter it. This one example is pretty much the only reason that I say (at the top of this article) that I'm personally torn on the issue of multiplayer time travel. Everything else seems to point towards time travel, but this one encounter seems to completely refute it all, and so I'm confused. Do any of my readers have any explanation or reconciliation for this? If so, I'd love to read it in the comments.
Human effigy and dried finger
There are also a few items in-game that make more explicit references to other worlds. The Human Effigy (DSII) and Dried Finger (DSIII) are prime examples.
"A warm, soft, shadow like effigy. Use this item to reverse Hollowing. It also weakens the links to other worlds, invasions and most cooperation.
Peer closely at an effigy, and one begins to perceive a human form, but whose form it takes depends on the person looking."
"Online play item.
Dried finger with multiple knuckles.
Use to strength connection to other worlds, allowing the summoning of a third phantom, but also a second dark spirit.
Also makes the summon of a dark spirit occur earlier. Use with caution."
But even the White Soapstone in first game refers to "other worlds" while also offering the explanation of distorted time. So if the words "time" and "world" are being used interchangeably, then such item descriptions don't contradict the possibility of time travel being the central mechanic of the multiplayer system.
Time travel is incontrovertible, but not necessarily for the multiplayer
In the end, time travel is a reality in the lore of the Dark Souls series. The stories of Oolacile, the memories of the giants, and the Untended Graves are all very explicit examples of this. The only thing that is in question is whether time travel is also the explanation for how the multiplayer mechanic works. If not, then these games employ both time travel and parallel dimensions! Seems a bit excessive. And the more and more I think about it, the more time travel seems to fit...