I like the concept, but maybe we switch it around and play the band of renegades (or....an ANTI guild! Dun dun duuuuuuuunnn!)? You know, the open minded humans and spirits working to take down their oppressors! Just a thought. 

I'd be cool with it the other way too, as long as the spirits that were branded weren't treated that badly. How big a mark are we talking? The forehead is prime real estate and even though we all know I'm a sucker for those face tattoos, even I have my limits. XD