ToME: the Tales of Maj'Eyal

The moment I select the possession talent it goes epileptic mode on me.

Now, I am not sure if this is a bug or intentional so I didn't want to bug report it.

In any case... I can select possessor as a class and play it normally, but this won't work. Ideas? Answers? Please help.

That sounds like a bug that's been encountered before. Basically, several talents in the Possession tree are coded on the assumption that you will already have a point in the Bodies Reserve talent and thus the necessary internal bodies storage infrastructure will be present; that's true for Possessors but not necessarily for Adventurers. My Possessor Tweaks addon attempts to patch over some of those; as a stopgap, if you carefully avoid mousing over any talents in the Possession tree before you've put a point into Bodies Reserve, you should be okay.

Thank you for the detailed answer!