So recently Dead Simon one of the rare mob, puzzle and artifact developers posted on the official forums that we were off-base about how rare spawns actually worked over there in the achievement and collections forum. Someone then suggested that perhaps rare spawns were not set by placeholders, or completely random, but set off by triggers. After encountering this mob, I feel this could be the case.
I had just finished the last part of the Hazeed chain before getting the quest to kill Hazeed. I had been on the hill where he spawned at least 6 times prior to seeing him going back and forth finishing the chain. It wasn't until after I obtained the quest to kill Hazeed that he spawned. I find it interesting, and a strange coincidence that in his dialog your reply to him is that, "Prince Hazeed will fall!" right before you actually go off to kill him. So my theory is someone completing this prior section of this chain (which I did not mention you actually kill 3 other council members...hmmm) causes him to spawn.
Now I do not know if there is a timer between spawns where two people doing the chain consecutively can't spawn him right after one another, but you'd have to assume so.
Edited, Apr 7th 2011 5:51am by Ailurion