For every hero, there is a villain. For every light, there is a shadow. Every rose has its thorn, but if you would just pour some sugar on me ... erm. /cough. The point is that we all have our nemeses, from Hogger and Mor'ladim to Fel Reaver, Illidan, M'uru, and the Lich King himself. Maybe you got stomped too many times by the devilsaurs in Un'Goro Crater, or maybe Mimiron had your raid frustrated for weeks on end. For my guild, it was Sindragosa.
When we got into ICC late in Wrath's life cycle, a new, small guild with a mixture of experienced raiders and players who had never ventured past heroic dungeons, we breezed through the first two wings. Blood Queen gave us a bit of trouble, but we soon swept past her and Dreamwalker and found ourselves facing off against the enemy from our loading screens, the last stop before Arthas himself. We went over the strategy, practiced our tomb placement, set up flares, and gave her a try. And another. And another. And yet another.
Though we were all sick of hearing her voice, for some reason, we just couldn't get Sindragosa down. We had trouble with disconnects, lag, Blistering Cold, multi-tombs, tanks getting tombed, clearing stacks, and pretty much everything else imaginable. The whole guild was aching to slay that dragon, and when at long, long last we managed it, the jubilation was unlike anything we'd had before. I doubt even the Lich King's demise would elicit the kind of cheers I heard on Vent when she went down. Our nemesis was vanquished at last.
Which boss or creature was (or is!) your greatest enemy in WoW? How long did you work on beating it? How did you celebrate when it was finally over?