Where is Wither.

Wither, the Elite Necromancer Curse...I'd love to have it to hit up those monks. Where do I find it? I've been asking/looking everywhere for over a week. All of the guides say it is in Druids Overlook, but there's not even a skill trainer there. Where is Wither? I hear I'll have to "capture" it from a boss, if so where is this boss?

Moved to Arcane Repository (skill discussion)