Players will be able to take class levels after trainers are unlocked.  Trainers are unlocked by completing adventures that involve that trainer as related to your background.

Prestige Classes may be chosen from any pathfinder approved source and can be trained by an NPC who would likely know about that particular prestige class. This must be okay's with the DM first.

Classes in Abaddon:

Barbarian

Death Mage

Dragon riders (Locked)

Fighter (Locked)

Oracle

Puglist

Ranger

Rogue (Locked)

Thief-Acrobat (Locked)

Shaman

Shadow Assassin

Sorcerer (Locked)

Summoner (Locked)

Warmonger

Unknown (Locked)

Unknown (Locked)