I finally made it to the hidden temple in the bottom of the goblin's lair. After 6 attempts starting over due to server resets, server crashes and being defeated once. I'm in the hidden temple, there's no more goblins to be seen. I use the portal and appear back outside the dungeon with no completion. I looted the chest and killed every goblin. I missed something, but whatever it is it was very well hidden.

There is currently an issue with the spawntrigger for that boss, which has been documented in the IA forum. A fix is set to go in with my next update.