I poisoned all the wells, enganged Thoraskus and he vanished. I killed the snakes, got to the treasure room and thoraskus never reappears in the 'locked door room'? i have tried invisibly purge but he is not showing up anywhere. any ideas?

I know it's almost a month late but I'll give an answer anyways.

Thoraskus retreats when you have dealt enough damage so what you have to do is prevent him from escaping. What I do before facing Thoraskus is summon 6 summons and surround Thoraskus along with any melee fighters my party has. Ignore the snakes and just focus on Thoraskus. Even doing this take a bit of luck but when he starts to run, it's likely your summons or his own snakes will block his escape route. If he escapes, then it's time to reload and try again.

Not sure if there's an easier way but I've had moderate success with this method.