As long as +Justin Isaac keeps posting cool public domain creatures over at Halls of the Nephilim, I'm going to keep stealing them for the monsterfighting pits. Here's my mod of the monster he recently presented with the awesome '50s horror story "Demons of Docar"...
(And now I see that the demons also got an excellent and extensive treatment a while back on the Swords & Stitchery blog. You can check that out here!)
I'm a little tempted to treat "Demon" as a worthy enough subtype to get recognition on these cards...kind of like how they're treated in Hearthstone:
There are only a few "creature types" that get specific mention in Hearthstone: Demon, Murloc, Beast, Dragon, Mech, Pirate...that might be it. I could see taking a similar approach. For now, though, it seems sufficient to let demonic entities deal damage of the Void/Aether type.