Darkins

Overview

"We had another name once... Now, we are Darkin."

— Aatrox known as "The Darkin Blade" or "The World Ender"

The Darkin are stated to be a collection of sentient weapons by Eduard Santangelo and the Shadow Order, of which only three have been revealed in modern times so far.
In truth, the Darkin are corrupt Ascended who were imprisoned within their own weapons in an act that ultimately ended the Darkin War. The nature of their corruption is implied to be self-inflicted after the mental scar left from the horrors of countless wars and those witnessed in the Void War, finally taking its toll on them centuries later with Azir's death during his Ascension and the subsequent fall of Shurima, leaving them feeling purposeless, betrayed and forgotten by the world they once swore to protect to which they gave their mortal lives to fight for.
In the centuries since, a handful of these weapons have been rediscovered by the ignorant and the ambitious alike.

Physiology

The Darkin began their story as revered human warriors of Ancient Shurima, Ascended into serving against the threat from the threshold. After finally halting the Void's perverse incursion, the surviving Ascended entitled themselves as "Sunborn" as first witnesses of a dawn of peace they brought to a now safe world.
Centuries after an age of war with the Void, despite their divine makeup, their minds remained as frail as the mortals they once were. When the light of Shurima finally left the world, many of the immortal Ascended found themselves lost. With no emperor to lead them, these “darkin” eventually fell to warring among their own kind—and the peoples of Runeterra were caught in the middle.The ceaseless warring and the horrors they witnessed slowly took their toll, leading them to fight over who was worthy of taking over the world that forgot them. This started the War of the Sunborn, known better to the world as the Darkin War.

The laws of mortal life held little meaning for the darkin. Over the centuries, they mastered many forbidden forms of primal magic, crafting their own flesh and armor with equal ease, until they were completely unrecognizable as the noble warriors they had once been.
During the Darkin War depictions of the self-corrupted Sunborn, now known as Darkin, as already having a demonic appearance—the result of using blood magic, fueled with each mortal and Darkin life they took, to reshape their own bodies and armor and meld them as one.
Toward the end of the Darkin War, the Darkin were imprisoned within their own weapons and wielded against their brethren until all who remained were likewise trapped. Those who held a Darkin would gain a portion of their power, but through a host was also the only avenue through which a Darkin to gain a mockery of freedom. Even when able to wrest absolute control of a body, the power of a Darkin is too much for any mortal body to contain - condemning the Darkin to a fate of constantly harvesting life force, eventually completely depleting the mortal's energy and being forced to seek another host, lest they be rendered inanimate once more.
To date, a golden-armored warrior queen is the only being known to have held a Darkin and not succumbed to its will. That said, her final act after dispatching all other Darkin was to trap herself and the Darkin Bow within a well in Pallas, condemning herself but denying the Darkin freedom as well.

Known Traits

As an Ascended is created, a celestial concept merges with their mortal body and gains them ascended power.

When a Darkin is trapped, that concept is trapped/deleted. Humans no longer could even have the thought/idea of what the Darkin was the avatar of.

Destruction! Wrath! Fury! As it was meant to be.

— Rhaast, the Darkin Scythe

From the moment a darkin weapon chooses a host, that mortal will find themselves compelled by a power they can scarcely hope to master—no matter what seductive whispers they might hear in their mind. That power will soon take root in flesh and bone, reshaping them into a new, monstrous amalgam.
The weapon's wielder and any Darkin trapped within seem to enter a symbiotic relationship, the wielder gaining a portion of the Darkin's power and the Darkin in turn gain some agency and a venue of 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 extirpate the Darkin's persona to gain full access to the weapon's power. In both cases, the dominated other is described as "dying."
Though the darkin's hunger for vengeance is inhuman and eternal, they must overcome the limitations of their new form if they are ever to sate it. Memories of who they once were, what they aspired to be, and what they have become, all now blur together in their flawed, mortal minds.