Ahriman

Ahriman - known also as the Undead God, the One That Ends and the Defiler of Death - was one of the twelve Dark Lords that governed the multiverse before Mankind, and a source of all death-themed magic. Certain elements of his story are known, however those are mostly shrouded in myth and heavily painted by his own insanity - as a result, to say that this story is subjective is an understatement of a century.

Physical Description

Body Features

It is unknown how Ahriman actually looks like. The Dark Lords and the most powerful of their servitors appear significantly more aligned with the human minds - with, for example, Ahriman and his servants appearing to be rather death-themed - however it is not their ACTUAL look. It's just a self-censorship that human minds enforce upon themselves, seeing them as the closest approximation that they could handle.
In case of Ahriman, his 'false-look' made him appear as humanoid of approximately human size - an actual exception among the Dark Lords, who mostly appeared much larger. He was said to have worn a robe made of screaming faces (among which people recognized their previously dead loved ones), which is the only universally certain part of his description. Everything else varied greatly.
Due to his small size, there are some scholars that actually doubt if it was truly Ahriman, and suggest that the Black Sun itself might have been the actual Dark Lord. If so, then it going underground and staying silent for centuries right after the 'Ahriman' fell is highly unusual for the Dark Lords.

Special abilities

Ahriman was a master of undeath. He didn't join the battle until his citadel came under siege, instead he produced more and more horrible undead abominations to send against the invading humans. In a way, his ability to adapt to the circumstances he was facing was among his most powerful abilities - and one that most other Dark Lords lacked. Eventually, however, he was cornered.
When that happened, he joined the fight, casting curses from the top of his tower that has turn tens of thousands of soldiers into hostile undead, and weakened others to the point where Ahriman's armies managed to overtake them. What took him down was Mankind's bringing back its soldiers and sending undying machines and automatically-driven Universal Assault Units against him, something to which he had no answer.

Mental characteristics

Personal history

According to some reliefs and books discovered in the ruins of Ahriman's fortresses, Ahriman has once departed to defile the very concept of death itself. Surprisingly, he has succeded - but his own people turned against him, fearing the enlightenment and utopia he offered them. When the war ended, he was the sole survivors among the legions of death... which didn't hamper him in the slightest. And was, in fact, preferable to having to bother with the sapient life.
Ahriman continued to seek the end of everything, destroying numerous worlds, and become a Dark Lord in its earnest. He also did something to the star of his world, but it is unknown what exactly it was - the details are scarce and cryptic. However the result was his masterpiece - a world where undeath was a natural state, and both death and life were foreign entities.
He was eventually invaded by the human armies, who have forced their way through his armies, laid siege to his fortress and finally changed it (and Ahriman himself) into the Glowing Sea - a basin of heavily radioactive liquid, a mix of Ahriman's magic, gamma radiation and materials melted into liquid and never allowed to turn back due to being soaked with so much magic.