Ascended/Darkin

The Darkin are stated to be a collection of sentient weapons by Eduard Santangelo and, of which it has been stated only three are known to exist. There are—in fact—five.

However, a Rioter involved with the story described them as immensely powerful beings of which only the known ones were imprisoned within weapons long ago, and that only among those imprisoned has escaped confinement. The other known Darkin are and. The remaining two Darkin are stated to be unknown even in-universe, and even isn't sure of their survival.

History
The origins of the Darkin, and any history pertaining to their creation or imprisonment, is unknown.

The entity known as is known for appearing in wars waged by humans, coming forth when victory seemed assured for one side, only to turn the tables with a few words and an unknown power, which inspires a monstrous bloodlust in people. His influence in historical conflicts spans thousands of years, with his oldest known appearance being in a battle between the Protectorate & Magelords. He has also appeared in Ancient Shurima, Ionia, Noxus, Demacia and more recently Freljord. His work earned him dedicated devotees from various factions across history, being heralded as a hero to many of them, but he disappears as quickly as he comes, giving no indication of his true purpose.

was sealed away inside the Pit of Pallas in Ionia, before he was released by during the Noxian Invasion of Ionia. This Darkin personifies Vengeance.

The fate of varies from game-to-game: either killing  to become dominant over his body, or being destroyed by, based on the outcome of  Quest.

Known Traits
The wielder of those Darkin trapped within weapons seem to enter a symbiotic relationship, gaining a portion of the Darkin's power in exchange for giving the Darkin agency and an avenue for freedom. However, the Darkin can parasitically dominate their wielder, thereby annihilating the wielder's personality while in turn freely affirming their agency; similarly, the wielder can dominate and expel the Darkin to gain full access to the weapon's power. In both cases, the dominated other is described as "dying."

has been confirmed to have usurped his wielder long ago and it is known that actively seeks to gain control over  body, but  is either more patient or is content to share, as  refers to him as kin but  is known to be the wielder, and not the Darkin.

In regards to Varus, and the Darkin bow I think it's possible that the bow cannot consume the wielder's mind and body the same whay that the others can. For example, if Rhaast consumes Kayn, he can still go around murdering and satisfying his murderous desires, in contrast, the bow cannot consume Varus because he has no drive for vengeance. the noxians who wronged Varus didn't wrong the bow so without Varus's mind which has the thirst for vengeance, the bow would not have a drive and then, well we don't know what happens if Rhaast doesn't kill but I suspect he would lose power and control of his host, without Varus's thirst for vengeance i think that that would happen to the bow as well.

asserts that will die if Kayn lets go of the scythe, although the implication is that the Darkin's power is lost if a Darkin is killed in this fashion.

Trivia

 * The two factions within Aatrox's lore, the Protectorate and the Magelords, are highly likely to be the two factions represented on Summoner's Rift.
 * A stone mural can be seen along the north edge of the map that depicts two factions surrounding a crystal, with a small carving of visible to the left.
 * With release, it was believed that the Darkin were a race of demon-like beings. However, in  and  teaser it was stated that the Darkin were a collection of sentient weapons. With  release, a Rioter stated that the Darkin are powerful beings and that only the known Darkin were imprisoned within their weapons. He later emphasised known to players.
 * Based on the original premise, it was speculated that and  might be the two known-to-history Darkin that are yet not identified to players. This was supported by their winged-humanoid appearance in conjunction with the common Angels and Demons trope, as well as the promise that their out-dated and non-canonical lore would be updated in the future to something more in-universe. However, the other known Darkin were revealed to be  and.