After examining a live Dark One in every way we could devise, we failed to establish any sort of contact, exorcise the demon, or find any useful info at all. Being such a diligent person, I've ordered butchering of its fresh corpse, and finally we got something. Beside normal human-like organs, the insides of Dark Ones are filled with a gelatinous substance we dubbed 'demonic essence'. After death, this stuff quickly breaks down, creating that foul stench that forces us to use so much space to store the Dark One corpses. The carcasses themselves are useless, but I think that with proper sorcery, the 'essence' could be harvested from freshly slain Dark Ones - it HAS to hold some dark power... Thus I recommend we keep any caught Dark Ones in my dungeon for now, hmmm...