Was just farting around in the FAQ and came across this. As a rule I don;t play casters so I had scanned over it before. But now that I read it I thought it was a great it!

"What is soul corruption?
Soul Corruption is a system in which contact with fell powers has a number of effects on your character. Since mages must bargain with these powers for the sake of their abilities, they are the most susceptable to it. Some, such as the Herald of Xotli, have much to gain from it, others such as the priest of Mitra (whose power comes from the favor of a benevolent god who doesn't... appreciate demons for some reason) will need to purify any taints as soon as they can. Regardless of class, there are serious consequenses to gaining too much soul corruption, ranging from your character going "insane" and blasting everything around him to his soul being stolen down to the depths of hell."

How do you get it?

"spellweaving?
Spell weaving is where a character can take a magical ability,and "stack" others with it. For example, lets say a demonologist first casts a container spell. He then adds a fairly weak AOE shockwave spell meant to stave off opponents, a powerfull single fireball spell, and a on touch mana draining spell. Casting the final product results in an AOE blast of fire around the targeted area that drains some magic from enemies to the mage. Powerful abilities come at a cost though, fumbling such a spell can have disastrous consequenses for the mage and those around him, including massive amounts of soul corruption, even for those who don't use magic."

Being able to stack spells is a frikkin cool idea too. I must say Mages do sound pretty exciting.