This is a heavily community sourced theory, but I’ve been interested by the theory of Solaire being the lightning worm from the Smouldering Lake and the topic came around again in the Duckfeed.tv Slack with some even more interesting insights, so I couldn’t help but make a record of it. Special thanks to @federally for certain connections and details.
The original Dark Souls was well-known for its establishment of the foundation of the Dark Souls world. It’s also known for its spectacular character stories, both those that are drawn from sparse materials and those that are clearly established through gameplay (Siegmeyer). Without providing too much direct text or reference, many conclusions can be drawn from the material that the creators do provide. One of the most prevalent stories from Dark Souls 1 revolved around Knight Solaire of the Warriors of Sunlight covenant, and the rumors that he may be Lord Gwyn’s firstborn son stripped of his deific status and stricken from the annals of history.
However, after Dark Souls 3’s release, this popular theory lost credence rapidly; the primary reason for this is that Dark Souls 3 includes Gwyn’s firstborn as a playable boss battle, pitting the player versus the Nameless King and his stormdrake mount known as the King of Storms. This leaves the community with more questions. If Solaire isn’t the firstborn of Gwyn, then who is he? The Sacred Oath miracle provides some insight into that question: “This is the tale of the Sun’s firstborn, his faithful first knight, and the brave dragonslayer who served them both.” This miracle’s description seems to indicate that three characters were very close in the past. The Sun’s firstborn is the Nameless King, with very little doubt. The brave dragonslayer likely refers to Ornstein, who is almost certainly the best known dragonslayer that is introduced over the course of the series. And this leaves us with the faithful knight; some would conclude that this knight could be someone else, or perhaps that this description only refers to two characters with the Sun (Lord Gwyn) himself being referred to as the other half of the “both,” but it’s probable that the mantle of “faithful knight” would go to Solaire. His affability and love of jolly cooperation won much affection with the fans of Dark Souls, so why not with his liege as well.
One big question down. Now, if Gwyn’s firstborn and Ornstein are still around in some way, then what’s become of Solaire? Yes, he’s just a regular Undead in Dark Souls 1, presumably unlike the Nameless King, but there is an interesting theory about Ornstein not only surviving the events of Dark Souls 1 but also becoming another creature entirely(more on that later), so perhaps Solaire’s fate is similar. In Dark Souls 1, two fates can befall Solaire: the first is that he falls prey to a Sunlight Maggot in the Demon Ruins while questing to find his own sun, and the second is that accompanies you to the Kiln of the First Flame as a phantom, implying that he’s also there to fight Lord Gwyn and kindle the First Flame. In the first scenario, a vulnerable and depressed Solaire joins the player at a bonfire, admitting that he’s unsure that he’ll ever find his own personal sun. Later in the area, Solaire can be found in a corridor with maggots surrounding him, one of them embedded in his head and glowing brightly. He’s abandoned his quest and gone Hollow, believing that he’s found his sun and will gladly betray you in order to keep you from stealing it from him. The player is forced to slay their old friend and move onwards alone. However, it is also possible to reach the corridor well before him and kill the maggot that would drive him mad, allowing him to venture onward unharmed.
With the background info out of the way, we can now really look at the question. In the first situation, Solaire’s corpse is left in the Demon Ruins, likely still nearby the maggot nest. In Dark Souls 3, the community posited a strange theory that relates to this scenario; perhaps Solaire has been consumed by the maggots, causing them to adapt lightning magic/abilities and have an affinity for lightning. This theory only has minimal support, but it’s an entertaining one. The worm uses a lightning attack, drops a lightning miracle (although not one that is associated with Solaire in particular), and drops an Undead Bone Shard that could possibly be the remains of Solaire. In the other scenario, which is widely believed to be a noncanonical version although it makes for a more sound theory, Solaire was in the Kiln of the First Flame in his own world, putting him in a position in which he could defeat Gwyn himself and kindle the First Flame. By doing so, he would become another part of the Soul of Cinder, the final boss of Dark Souls 3 and an amalgamation of all of the Undead and Lords of Cinder who have previously kindled the First Flame. In either scenario, Solaire’s memory lives on in Dark Souls 3, preserved over the centuries within the confines of other creatures and constructs.
More on Ornstein and the Nameless King: It is commonly known that Ornstein’s armor can be found in Archdragon Peak, in an area that is sealed off until the Nameless King is defeated. The location of the armor set may be a coincidence that occurred while the creators were simply dropping the armor into the world to widen the choice of armor in the game, but when taken into consideration alongside the fact that the Ornstein in Anor Londo during the events of Dark Souls 1 may have been a mere illusion conjured by Gwyndolin, as well as the possible close relationship between Gwyn’s firstborn and the famous dragonslayer,it becomes plausible to conclude that Ornstein has become something else. After the Nameless King’s change of heart regarding dragons, perhaps he and his faithful dragonslayer partner left Anor Londo quietly to take up residence with the dragons. The Nameless King, already a god of war by his family’s standards, becomes a king among the dragons as his partner renounces his traditional dragon hunting spear and bronze armor in order to transform into a mighty dragon-like being, the King of Storms. It’s not guaranteed by any means, like much of the possible lore theories in Dark Souls 3, but it is another interesting theory to cap off this discussion of the three famous warriors of Gwyn’s long-dead kingdom.
Thank you for bothering to read all of this and I hope that you enjoyed your time!