But hey, on a subject that may be just as divisive, why do they feel the need to keep resurrecting dead characters to be raid/dungeon bosses? Having Mannoroth come back in BC pissed me off to no end. Way to completely invalidate Grom's sacrifice.
It's been mentioned, but you are thinking of Magtheridon, who is a different Pit Lord.
Mannoroth was the Pit Lord that gave his blood to the orcs originally, and was killed by Grom Hellscream, his bones taken and displayed in Orgrimmar since WoW began. Garrosh moved his skull into the new Grommash Hold after Cataclysm, cut off parts of his demon tusks, and now wears them on his shoulders as a trophy of his father's past victory.
Magtheridon was the Pit Lord that, after Draenor blew up and became Outland, muscled his way into the broken dimension to take control of it. He ruled over Outland for years until Illidan appeared and used his forces to siege the Black Temple and oust Magtheridon from power. Instead of killing him, Illidan gave the demon over to some of Magtheridon's former subjects, the Fel Orcs, who drained him of his blood and infused it into themselves.
As for characters getting ressurected, well you can sometimes blame the fanbase. At the last BlizzCon, a fan asked Metzen if they would ever bring back Illidan. He made it known they had no plans, but asked what people thought of them bringing Illidan back, to which the crowd cheered. Metzen followed that up with a "Ok, point taken, done", and so pretty much at some point we are going to have Illidan around again, maybe a raid boss maybe an NPC, who knows.