UPDATE SEPTEMBER 1, 2016:
The release of Dark Souls III has finally answered this question. That game has rendered the speculation below completely moot and invalid. However, this post will remain preserved for posterity.
There is considerable debate within the Dark Souls fan base regarding whether or not Knight Solaire of Astora may be the forgotten son of Lord Gwyn, and the God of War. The game contains many references to a forgotten God of War, who was the son of Lord Gwyn, the God of Sunlight. The primary source for this is the item description for the Ring of the Sun's Firstborn:
"Lord Gwyn's firstborn, who inherited the
sunlight, once wore this ancient ring.
Boosts the strength of miracles.
Lord Gwyn's firstborn was a god of war,
but his foolishness led to a loss of the
annals, and rescinding of his deific status.
Today, even his name is not known."
The game doesn't specify what the God of War did to be stripped from the annals, but the blunder cost him dearly. As a punishment, Gwyn and the other gods rescinded the God of War's diefic status and expelled him from Anor Londo.
But it didn't end there. Based on the content of the game itself, it appears that the gods also removed all references to him. This included removing or destroying any statues depicting him and redacting his name from records. Both his name and face are lost to history, as is his fate. Was he cast out of Anor Londo? Was he made mortal? Was he cast out of the world, entirely?
The gods made an effort to eradicate all records and traces of the God of War.
Statues of him were removed or destroyed all throughout Anor Londo and Lordran.
If the Warriors of Sunlight covenant (lead by the God of War) existed prior to the God of War's expulsion from Anor Londo, then it would also stand to reason that the gods disbanded the covenant. It's highly unlikely that gods would have permitted the covenant to continue to function, since its followers would be able to continue to teach of the existence of the God of War. These followers would likely have been forced to renounce the covenant or become an underground cult. The fact that the covenant still exists and has followers is likely due to the waning influence of the gods after Gwyn left to link the flames and the gods were forced to abandon Anor Londo.
The Sunlight Medal does offer one clue as to the fate of the God of War and his covenant. It suggests that the forgotten god still lives, and still watches over his followers:
"This faintly warm medal engraved with the
symbol of the Sun, is the ultimate honor,
awarded to those who summon the Warrior
of Sunlight and complete a goal.
The symbol represents Lord Gwyn's firstborn,
who lost his deity status and was expunged
from the annals. But the old God of War
still watches closely over his warriors."
In any case, we meet one NPC Warrior of Sunlight during the game: Solaire of Astora.
Solaire wants to be the sun
Solaire doesn't reveal much about himself. He is a Warrior of the Sunlight who has a strong reverence for the sun itself. In fact, he is on a possibly futile attempt to obtain his own "sun"; although, he never explains to what end. He also implies that he intentionally became undead in order to take on this quest. Whether he is seeking a specific item, or if he is simply looking for any "sun" is also not explicitly clear. All we know is that - depending on the player's actions - he may find a suitable sun, but it will drive him mad.
Solaire supposedly comes from Astora. The Blacksmith Andre, Firekeeper Anastacia, and Oscar (the knight who helps us escape from the Undead Asylum) all hail from the same land as Solaire. But Solaire doesn't talk about his past or his homeland. In fact, the only things he seems to want to talk about are his devotion to his covenant and his quest for his own sun.
Solaire is technically from another world, even though he doesn't appear as a phantom...
It is also worth noting that, unlike other NPCs with dialogue, Solaire is not actually in your game world. He is an adventurer from another dimension (or another time), much like the player phantoms that you may summon to assist you. He mentions that time is convoluted, and that he doesn't know for how long his world will be in contact with yours, but he offers his assistance as a summon.
And this is where Solaire starts to become interesting as a character. He is the only Warrior of Sunlight that we meet in the game, but he isn't from this world at all. This begs the question: are there any Warriors of Sunlight left in our world at all?
Parallels between Solaire and the God of War
So if Solaire isn't from our world, is he actually from Astora? He doesn't use any of the trademark equipment that we see used by other Astora knights (i.e. the Elite Knight set). So is it possible that his origins and training are from somewhere else? Could it be that he is the God of War, who still watches over his warriors, despite possibly being cast out of the world?
Hints that Solaire may have a secret past life begin coming in as soon as we meet him in the Undead Burg. One piece of evidence is his dialogue in the the first meeting in the Undead Burg:
"Oh, hello there. I will stay behind, to gaze at the sun.
The sun is a wondrous body. Like a magnificent father!
If only I could be so grossly incandescent!"
The simile may be a subtle bit of foreshadowing that associates the Sun (i.e. God of Sunlight, Gwyn) as Solaire's father. From a less meta standpoint, it may also mean that Solaire himself subconsciously sees the sun as his father, even if he doesn't consciously remember his past life and heritage. The final desire to be like the sun could also be symbolic of a subconscious desire to reclaim the deific status that was taken from him.
In a first playthrough, this line is easy to dismiss as insignificant, since we don't know anything about the game's extended lore, the God of War, or the Sunlight covenant. Solaire just seems like a strange, eccentric individual. But once you've made it a significant portion of the way through the game, this line becomes more meaningful.
Solaire's dialogue, actions, and quest draw some implicit parallels between himself and the expunged God of War,
implying that he subconsciously remembers his formerly divine life and desires to return to it.
Dialogue later in the game invokes similar parallels between Solaire and the God of War. After visiting Anor Londo (but before progressing to the Demon Ruins and Lost Izalith), if you travel to the Altar of Sunlight, you will find Solaire, and he will say:
"Forgive me, I was just pondering… about my poor fortune.
I did not find my own sun, not in Anor Londo, nor in Twilight Blighttown.
Where else might my sun be? Lost Izalith, or the Tomb of the Gravelord… ?
But I cannot give up. I became Undead to pursue this!
But when I peer at the Sun up above, it occurs to me…
What if I am seen as a laughing stock, as a blind fool without reason?
Well, I suppose they wouldn't be far off!
Hah hah hah!"
Similar to his earlier dialogue regarding the sun being a "magnificent father", this dialogue also draws parallels between Solaire and the forgotten God of War. He peers at the sun and wonders if they [the gods, presumably] would see him as a laughing stock and fool for his failure to find his own sun. This mirrors what happened to Gwyn's firstborn, who was punished for foolishly causing the loss of annals. Again, perhaps Solaire subconsciously already knows that the gods are disappointed in him...
Is Solaire's quest for a sun a subconscious attempt to seek acceptance and vindication from the gods? Or simply to become like to gods; thus, reclaiming some of his former status?
Perhaps this also sheds light on the firstborn's mistake. If Solaire parallels the God of War's descent, then could it work the other way around too? Perhaps the God of War's "foolishness" was an act of hubris or an attempt to elevate himself to be equal to (or superior than) Gwyn himself. And in this act of foolish hubris, he either intentionally or accidentally lost the annals. Maybe he was literally trying to rewrite history.
In addition to having dialogue that parallels the God of War, Solaire's actions also have parallels. It is said that the God of War still "watches closely over his warriors". Well, Solaire does watch over the player character; although, it is worth noting that he does this regardless of whether you enter his covenant. His summon sign is the most common NPC summon in the game, and it occurs at many of the most important boss battles: the Gargoyles, the Gaping Dragon, Ornstein and Smough, the Centipede Demon, and even Gwyn himself! Sadly, you're on your own against the Capra Demon...
Solaire watches over his fellow Sunlight Warriors, just as the God of War is supposedly doing.
He is the only NPC that can be summoned for the final boss battle with Gwyn, and so he is the only NPC who is presumably able to make it this far (since Oscar's appearance at the Kiln was cut). Since Solaire is a hero from another world, he is possibly his world's "chosen undead", who manages to defeat Gwyn and link the flame.
If this is the case, and Solaire is Gwyn's firstborn, then there is a poetic irony here: the gods stripped Solaire of his deity status, only to have him be the one who fulfills their wishes and re-ignites the flame! On top of that, Solaire's divine strength and durability means that he has a strong soul, which will burn for a long time. It may even burn longer than Gwyn's did, considering that Gwyn's soul was weakened from sharing it with others (Seath, the Four Kings, etc.). In this way, Solaire would become the light of the world (which should surely count as becoming like the sun), and may even usher in a second Age of Fire that could surpass Gwyn's in length.
This is all assuming that he doesn't foolishly settle on the Sunlight Maggot as his sun and get himself killed by the player.
Solaire's equipment also connects him to divinity
But Solaire himself isn't the only one who is drawing parallels between himself and the God of War. The game world itself, and some item descriptions also create implicit connections between Solaire and the ex-god.
Solaire's armor says it is not magical or empowered in any way, telling us that all of Solaire's inhuman strength, durability, and prowess is natural talent:
"Armor of Solaire of Astora, Knight of
Sunlight. The large holy symbol of the Sun,
while powerless, was painted by Solaire himself.
Solaire's incredible prowess must have
come from rigorous training alone, for
his equipment exhibits no special traits."
Solaire's helmet, leggings, gloves, and shield all have a similar description. The fact that his equipment goes out of its way to emphasize that Solaire's prowess is not the result of magical enhancements is interesting.
If Solaire is merely human, then he is an incredibly strong human, training notwithstanding. The insistence that it "must have come from rigorous training alone" does seem a bit tongue-in-cheek, as if the writers didn't want us to believe it. Of course, if he were a former god, then his exceptional strength and vitality would make more sense.
But there are other clues in the world that imply that Solaire is more than he appears.
Is this a statue of Gwyn's firstborn?
At the Altar of Sunlight bonfire (below where the Hellkite Drake perches), there is a statue depicting a woman holding a young baby. There is no inscription telling us who this statue depicts, but the individuals shown are clearly significant. In addition, this statue's presence next to the Sunlight Altar dedicated to the God of War's covenant suggests that the statue may be honoring the God of War's birth. Perhaps the gods felt that removing any inscription was sufficient enough for redacting the God of War from history, since it would be unlikely that anybody would know who that baby is.
But the statue does leave one important clue: the baby in this statue (presumably Gwyn's firstborn) is holding the Sunlight Longsword - Solaire's sword.
"This standard longsword, belonging to Solaire
of Astora, is of high quality, is well-
forged, and has been kept in good repair.
Easy-to-use and dependable, but unlikely to
live up to its grandiose name."
This sword is unique within the game. It is only dropped by Solaire and cannot be acquired through any other means. It also has a unique model, and varies slightly from the standard Astora's Straightsword. Other items that are dropped by specific characters or enemies have descriptions stating that the item is standard equipment for a group, but Solaire's sword description does not! Noting that the sword has been kept in good repair also implies that the sword is old and / or well-worn in battle.
Is this the very same sword that is depicted in the statue? A statue that seems to be honoring the birth of the God of War. This seems to be a pretty concrete clue (pun intended).
Are there any alternative characters?
While Solaire seems to be the prime candidate, there two other significant candidates for the God of War.
Blacksmith Andre of Astora may have been the intended firstborn
Andre of Astora is probably the closest runner-up. His size, strength, and physical resemblance to Gwyn (i.e. he also has a white beard) have led some players to theorize that he may be the God of War, rather than Solaire.
The white beard is probably a relic from a time during the game's initial design stages in which Andre was intended to be a descendant of Gwyn (though not necessarily the firstborn God of War). This is explained in an interview with lead designer Hidetaka Miyazaki contained within the Dark Souls Design Works. The interview also explains what Andre's role was supposed to be: he was intended to be protecting a goddess statue in Firelink Shrine that was originally intended to conceal a path to a necessary stage of the game. At some point, Andre would move the statue aside, allowing the character to progress with the game. The statue in the concept art appears to be the same statue as the one at the Sunlight Altar bonfire, but it could have been originally located in Firelink Shrine, or in Andre's room below the Undead Parish. Presumably, the passage would lead to the Catacombs, Darkroot, or Kiln of First Flame. In any case, protecting the statue may have been part of his exile punishment.
"He was originally a descendant of Gwyn whose task it was to protect a door within the fire link shrine. In the end he was going to push aside the goddess statue to let you progress, but as development progressed he became just a simple blacksmith."
- Hidetaka Miyazaki
He also has the ability to forge divine weapons that scale with Faith if given the Divine and Occult Embers, implying a possible relation to the gods; although, this is very speculative. His role as the primary blacksmith makes him an important NPC, and his services are practically required in order to progress in the game. He is also somewhat protective of the player, and shows concern for your well-being. In that sense, it could be said that he is "watching over you", but just like Solaire, he has this dialogue regardless of whether the player joins the Sunlight covenant.
Speaking of Andre's dialogue, he also happens to be the only NPC character (that I am aware of) who has actual lip-syncing for his dialogue, which goes to show just how important his role in the game was originally intended to be.
His dialogue also makes reference to weapons not betraying you, implying that he may have been betrayed as part of his backstory. This may be a reference to him feeling betrayed by the gods for expunging him, if he is the God of War. But since he never elaborates on his history, we have no idea what this betrayal refers to.
The Great Lightning Spear does offer another hint:
"Miracle passed down by
those bound to the Warrior of Sunlight covenant.
Hurl giant lightning spear.
The weapon of the God of War, who inherited
the sunlight of Lord Gwyn, but had
respect only for arms, and nothing else."
It suggests that the God of War had the power of sunlight, but that he preferred to use weapons instead. Andre is definitely quite reverent of weapons, and even suggests that the player should "respect" your weapons. But this could shed some light on his betrayal remarks. Perhaps he wasn't betrayed by a person. Perhaps his magic or faith "betrayed" him! If he was the God of War, and he had the power of Sunlight Spear, then perhaps he tried to use it in battle, but it failed him. And perhaps that failure lead to the loss of the annals and his eventual exile.
Solaire, on the other hand, chucks Lightning Spears around like dollar bills in a strip club. So if he didn't have respect for miracles when he was a god, he certainly has respect for them now!
It seems very likely that Andre was originally intended to be the forgotten firstborn son of Gwyn in an early stage of the game's development (assuming that there even was a forgotten firstborn son in those early drafts), and that some of those clues remain in the final product. However, in the final product, his connections to Gwyn are almost non-existent. We also don't know whether Solaire was present in the game at this stage of development, or if his character was significantly altered. It could have been that Solaire was designed (or redesigned) to have the parallels to the God of War after it was decided that Andre would be just a regular blacksmith.
The Blacksmith Andre of Astora is a uniquely large and strong NPC with a physical resemblance to Gwyn,
and his dialogue shows concern for the player's well-being.
As it stands now in the final released version of Dark Souls, there just isn't compelling reason to believe that Andre is the God of War, as the best evidence in favor of this was deliberately cut by the design team. Is there another alternative?
UPDATE, MARCH 6, 2016: Oscar, Knight of the Undead Asylum
The other character that is a possible contender is the un-named knight who frees the player from the Undead Asylum. This knight is not named in-game, but text from cut content reveals that his name was supposed to be Oscar of Astora. In the final game, he has little relevance to the game's plot (other than setting the whole thing in motion by sending the player on your journey). He dies in the asylum and is never referenced again, except if you return to the asylum and fight his hollow phantom.
Oscar of Astora originally played a bigger role.
Oscar originally had a larger role in the game (much like Andre), but his role (and his very name) were both cut from the final product. Originally, he was supposed to appear in Darkroot Garden to help the player fight against the bandits and forest covenant phantoms. He would then appear at the Kiln of the First Flame and would challenge the player regardless of which ending you chose. If you placed the Lordvessel for Frampt, he would declare you a "pawn" and try to kill you so that he could become the Dark Lord. If you placed the Lordvessel for Kaathe, then he would decry you as a "fiendish Dark Lord" and attempt to kill you.
So, why would we think he might be the God of War? Well, he's a forgotten character within the game. He is unnamed in the actual game, nobody else in the game notices or references him. So there's a parallel to the God of War. He is also engaging on a pilgrimage to Anor Londo due to an "old saying in my family" about ringing the bells of awakening and learning the fate of the undead. He seems to have some inside knowledge regarding the bells and the undead's role in the world.
There's also the name, Oscar. The most common meaning of this name is derived from its Irish origins meaning "deer friend". But the name could also be derived from the Old Norse name Osgar, meaning "god spear"! That establishes link between Oscar and the Lightning Spear. This connection to the Lightning Spear, and Oscar's minimized role in the plot, combine to create a striking parallel between him and the God of War.
However, much like with Andre, it appears that any intended connection to the God of War may have been cut, as Oscar's very name was removed from the game, along with his extended questline. So again, we're left with a burning question about what the original intent might have been, but with no actual in-game evidence linking Oscar to the God of War (other than his "old family saying"). But then again, maybe this is the point? Oscar, like the God of War, is forgotten; his name has been stripped from the annals of the game itself.
Dark Souls II leaves the issue an open question
In the end, the identity of the God of War remains a mystery in Dark Souls. The case for Solaire is fairly strong, but it is not bulletproof. It is entirely possible that the God of War is simply not existent in the game at all, and the references to him are purely for lore.
Dark Souls II doesn't add anything to the table [that I'm yet aware of] except for a new pantheon and the name of a new war god: Faraam. The Lightning Spear miracle is also present in Dark Souls II, and its description only further obfuscates the issue:
"A miracle that launches a spear of lightning.
Said to be the legacy of an ancient clan
whose leader was revered as the God of Sun.
The name of the clan has been lost to time,
but the gross incandescence of our magnificent
father shall never wane."
Praise the sun!
It suggests that the leader of the Sunlight Covenant was a sun god, and not a god of war. This could be the result of revisionist history. Or it could suggest that the God of War became the Sun God after the events of the first game. Or it might not be talking about the God of War at all.
The "gross incandescence" of a "magnificent father" is a clear reference to Solaire's dialogue referring to the sun, so it's a safe assumption that the leader referred to is either Solaire or Gwyn. Perhaps this means that Dark Souls II takes place in Solaire's world, after he defeats Gwyn, offers himself as kindling, and becomes a new God of Sunlight? Hopefully that qualifies as meeting his goal of finding his own sun, but it doesn't say anything about his prior diefic status.
In any case, this question is unlikely to be resolved any time soon, and it remains an open topic of debate for the games' fans. I, personally, believe that Solaire is the God of War. But your conclusion may vary.
In the meantime, carry on with jolly cooperation, and praise the sun!