This mob spawns at the place of (not from the body of) the black reaver at the top of the elevator. The first one you see overlooking the elevator itself.
Had camped out the night before atop the elevator and logged in today to kill this guy, Dropped Jade Reaver, the essence, and a complex platinum gem dusted rune.
Spawns in the area with the blue floating stars overlooking the overhang. He follows the spawn rate of the reavers, so you will get either a reaver or him ever 2 hours. This part of the quests for shield took a little bit a time because it seems more Reavers spawned. Could be a long camp.
For me, he spawned in place of (not from the body of) the black reaver at the top of the elevator. The first one you see overlooking the elevator itself. I had cleared the reaver and waited for the respawn. To my surprise, Tzirathk popped instead of the reaver.
Well, i was killing Black Reavers trying to get a jade reaver to drop, and our grp runs into this guy. We decided to kill him, and wat do ya know... He drops a Jade Reaver! I guess thats one way to get it... =)