Hm...



...Nope, not quite. The Prince retained control in both forms. Curses!

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More constructively:

I like it, for the most part. The part about the ability transfers could use some cleaning up, though. First off, it should be its own (Ex) or (Su) ability. Second, it could be worded in a manner that uses more official terms (eg "The Dark Mind can take X points of ability burn to an ability in exchange for a +X unnamed bonus to a different ability. The ability burn goes away after...") Lastly, is it only available to the Dark Half, or to the host as well?

I'm also curious as to whether or not either side can voluntarily cede control to the alter ego. The host can always activate his demonic body and voluntarily forgo the saving throw (say, to avoid taking responsibility for his actions, or because he's "not strong enough" to "do what must be done"), but what if the Dark Half feels like putting the host back in the driver's seat? That sort of thing would be invaluable for subtlety and corruption: For instance, the Dark Half could force his host back into control after placing him in a Paladin Trap (that is, a situation contrived so that any action, including inaction, will appear to be Evil) to "teach you a lesson" about morality, or it could even pretend to be a benign symbiotic force that only takes over when its power is needed (at least, until it's ready to launch the Master Stroke). Plus having access to a second set of class abilities is nice.

Lastly, is the host aware and conscious during the Dark Half's period of control? And, if so, does the detect thoughts spell target them both, or just the guy in control?