Split the class into three styles:

1) Reaver: once reaver status attained the AP travels as a spirit form til it can possess the fallen form of another adventurer or very powerful mob. It gets specific resistances and vulnerabilities no matter what form it has. Has to find a new fallen form each time the player logs on.

2) Reaper: an AP can spend the charges she/he collects to open up new powers related to the dark magics they wield. Additional effects for spells that are unknown to the AP at first but she/he will figure them out eventually.

3) Demonologist: AP collects "charges" in order to learn to invoke specific demonic attributes they can learn. Each attribute will come with certain advantages and disadvantages. Of course invoking too many attributes at the same time kills you and turns your body into a mindless demonic spawn that attacks anything it senses.

Had a bad dream about being hunted by a band of strangely empowered APs.

I really like these ideas. The most original, I think is the Reaver, whereas Reapers and Demonologists are more true to the class.

The Reaver is really awesome though, and it'd be interesting to see this implemented. It would add a certain amount of stealth to the class. I think it might be hard to code which mobs can be possessed and which can't, as well. I don't know how similar NPC coding is to PC coding, also. Can they be converted directly to PCs? Is that how Imms mobteract?

Splntrd