Fire, Force, Meta, Necromantic White, Temporal
When you reach the Void you'll discover that Arronax has had nothing to do with the events in Arcanum. In fact, he'll inform you that he's been imprisoned for over 2000 years, and that it must be Kerghan who is planning to return from the Void.
To get Arronax to join your party, you must free him from his prison. To do that, simply kill the spirit snake patrolling in front of his cell. You must add Arronax to your party if you want to talk your way past Kerghan at the end.