"We will crush you even if it takes another ten thousand years!"— Erelus to Volux the Forgotten(src)
Erelus, Forgotten Elder Angel, is a Super Unique angel found in the Battlefields of Eternity (Crag of Eternity part) in Act V of Diablo III. He only appears during The War That Time Forgot event, commanding the time-lost angelic regiment.
He is hostile to the player, thinking that they are a demon (most likely he was trapped in the stasis before nephalem came to exist). Erelus's wings, just like wings of his soldiers and Haziael, are black in color, apparently a result of long separation from the Crystal Arch.
In combat, he has Arcane Enchanted, Frozen, Fast and Teleporter affixes. Until killed, he will summon more and more Timelost Angels to join the battle.