the Pool of Spirits is straight west down the hall from the room with the squid guy...Xhorrox or something. Walk in the room and it should be an auto-update.
The step with benedictors and idols is in the the room with Xhorrox. The middle section of the wall of the tower in the room is climable. The idols spawn on each of the 4 corners on a little ledge that juts out off of the second level. They are gold and difficult to see sometimes because they blend in with the ledge. Each person has to destroy the 4 idols because the updates are not shared. The room repops pretty quick so the group I was in just camped and killed everything until everyone had destroyed all the idols. The benedictors spawn real quick too.
The next step requires you to kill some essenses (cant remember the exact name) in the Spirit Pool room. This is just west of the room with the idols. The essenses spawn in the middle of the room in a pool. You need 8 and there are initially only 6. My group cleared the room and then had to wait for a little bit before we could kill the last 2 essenses. Right after we killed the 8th essense Zarov the Soulsnarer popped. I can't confirm if this is an auto-spawn from killing the 8 essenses or if we were just very lucky.
Return to Thorn for your choice of bangflashers (stuns mob), vial of escape (evac potion) or a webbed cannister (stifle and slow I think). Thorn will then give you the quest for the 5th step for the mana tap weapons.