My theory.

Just wanted to say it first. With regards to the Darkin varus Aatrox and Raast I think a they have a brother and his name is Nightfaal. Weapons that slaughtered humans on the Battlefield using human to there own end.

One such weapon came across a Perfect candidate a Brutal warlord name unknown. when they were together they were unstoppable cutting swathes through throngs of humans, the Darkin began to exert itself upon the humans psychy This Human however was wary of the weapons influence and placed the Darkin in a metal case for long stretches. After years of isolation and forced pacifism the lid was opened and the warlord and a group of Wizards placed an ultimatum on the Darkin. #Obey me & Answer all my questions or be destroyed.

The Darkin began to divulge its secrets to the group of men. With thier developed power they began study and refinement of a Ritual. Nightfaal cared not,for finally after years of isolation he was upon the field of Battle once more. The Years had been kind to the Warlord he had obtained Wealth, Power and a Keep to match. Armies would approach the Keep and the warlord would ride out and cut them down,again and again it seemed year after year.

They fought many battles and the Darkin was glad of it although the weapon could sense an apprehension within the Warlord Something never seen before.

When a day came that the Largest army ever witnessed assembled against the Warlord Many times beyond what the Warlord could muster. After Council with his Wizards they Rode out to meet them:the Wizards worried, the Warlord apprehensive, the Weapon glad to soon have a new body to dominate.

The Warlord rode out to meet them as if it were just another day, Together they Battled long an hard against Insurmountable odds.

Fighting atop a Growing mound of dead enemies long into dusk his Battalion had died hours ago

NIghtfaal had never Drank so Heavily it was ectasy gorging on death brimming with Power awaiting his new Host. The Warlord took 8 Blades in his chest before he took a knee never to Rise again. Nightfaal called out to the 8, content for them to murder each other for his power

Something happened a Jerking Twisting Wrenching all Nightfaals power and fusing him with the corpse of Fallen Warlord.

TLDR. Nightfall = Nightfaal
A Darkin used now to fuel the Immortal undead nature of Mordekaiser