I was in a group that went after the Avatar of Agony. We had lvl 65 paladins (x2), shammy, cleric, monk, and lvl 61 druid. Cleric was able to lull Avatar with spell: Pacification.
The mob appears to be part of a placeholder system, but we found one up after a patch in the tower Axarerxius mentions below. The rough loc was p520, p228, n156 in the east side room. However that was just where he was for us, its possible he pops wherever there are 'constructs of agony', I'm not sure. We also got lucky and had him respawn in the same rough spot later.
Fight took about 2-3 minutes with a group of 58-65 including a 62nd warrior. Pre defensive the Avatar hit for about 583 max, with defensive he hit for 440 max and averaged around 249. He flurried 3 times over the fight, didn't rampage, but did enrage at low hps. We didn't see a gate attempt by it unlike some of the other mobs in camp.
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Voidslayer Jhereg, Destroyer of Worlds, Monk, <Celestial Navigators>, Drinal server Vanidor Silverbranch, Wizard, <Celestial Navigators>, Drinal server
This one spawns in the death pit. From zone in hug right and pass the first minni tower and you will come to a death pit. It's a death pit because one false move and you get 10+ adds.