Dungeon Slime

It lives in the Dungeon and turns up anywhere inside it once Skeletron is out of the way, though it is scarcer than the skeletons and casters around it. Every one carries a Golden Key and hands it over on death, which makes it the only repeatable key source in the game. If you have found more locked gold chests than keys, hunting slimes beats combing the halls for loose ones. It gives no gel, so do not treat it as a slime farm.

A Water Candle and a Battle Potion raise how often they show, but they raise everything else too, so bring gear that copes with a room filling up with Dark Casters and Angry Bones. A Hunter Potion is the difference between spotting one down a side corridor and walking past it, and a Lifeform Analyzer also flags them. One catch: with Royal Gel equipped they ignore you like every other slime, which is safer but means you go to them.