It hasn't spawned in over a week. I know its bugged before on both Mischief and Thornblade and someone had to repop the zone. I wish I could help with what bugs it but every time I've run the event its seemed fine for me.
Finally, the logging I add showed me where the problem is. I should have a speculative fix in the next patch. I re-popped after I was able to get the diagnosis.
