Imps are brown, thorny hominids, which take several bullets to kill. An imp can fry you with fire balls, or turn you into ground beef with their formidable claws. It's advisable to have something more potent than a pistol when taking on more than one of them. Demons are big, pink, and ugly. They're hard to kill, and have LOTS of teeth.
Spectres are similar to demons, except they are nearly invisible.
Knowing they're around is not a very comforting thought when you find yourself walking down a dark hall way. Other monsters you may run into include Lost Souls, Cacodemons, and the Barons of Hell. The most basic of these new demons is a former human which carries a chain gun and is called the heavy weapons dude. This guy is a real pain in the butt and never runs out of ammo. If you see one, it's time to retreat and regroup.