Dark Mage

The Dark Mage never spawns on its own. It only appears inside the Old One's Army, arriving on the last wave of a Tier 1 raid and turning up twice in Tier 3, on the fourth and sixth waves. There is no summon item for it beyond starting the event itself.

• It gathers a ball of dark energy and throws it straight at you. The shot travels in a line, so keep moving across it and it goes past.
• It stamps a rune into the ground that floods a very wide area with healing. Anything nearby with enough health that can actually hurt you gets topped back up, which can wipe out a minute of work on a tougher enemy. It cannot heal itself or another mage, so the answer is to kill the mage.
• It chants and scatters purple clouds over the solid ground around it, and Old One's Skeletons climb out of them a moment later. The clouds hurt while the skeletons are forming, so step off them rather than standing there swinging.

It floats a short way off the floor and cannot climb, so it stays on your level while the skeletons it makes go wherever they like. Lava and teleporters do nothing to it and traps land only half their damage, so the arena tricks that trivialize other events are weak here. Target priority is simple: the moment a healing rune goes down, drop everything and kill the mage, or the crystal loses ground while everything you shot recovers.

• A boss topped back to full keeps whatever phase it had reached. Santa-NK1, Plantera and Queen Slime are the exceptions and revert to their first form.
• The Moon Lord is immune to it.
• Very weak enemies and anything that deals no damage are skipped entirely.

In a Tier 1 raid the mage keeps coming back until the final wave is almost complete, which means a player in decent late pre-Hardmode gear can grind it for as long as that wave lasts. The catch is progression: killing the mechanical bosses or the Golem permanently bumps your world to higher raid tiers, so keep a second world you deliberately never advance if you want easy Tier 1 runs. The Gato Egg and Dragon Egg pets both come from here, at some of the friendliest pet odds in the game. The Creeper Egg is not his, that one belongs to the Ogre.