Chaos Elemental

It is a contact damage enemy that refuses to stay where you hit it. It sprints, blinks a short distance, and repeats, so anything with travel time tends to arrive where it used to be. It jumps to a random open spot roughly twenty tiles out, never right on top of you, and only into a gap tall and wide enough to hold it, which means a cramped tunnel limits its options considerably.

The useful detail is that hurting it locks the teleport for a moment, but only damage from you, your minions or your sentries counts. Lava and traps will chew it down without ever pinning it, which is the wrong way round if you were hoping to hold one in place.

Only on pearlstone, pearlsand and pink ice, and only in the cavern layer. No other hallowed block works: pearlsandstone, hardened pearlsand and hallowed grass all produce nothing. You do not have to be standing in the underground hallow yourself, just near enough to the right blocks, so a small imported patch is enough. Unsafe desert or spider walls behind those blocks shut the spawn off.

They carry an extra spawn check that catches almost everyone out. No town resident can be nearby, there must be no safe background wall behind your character, and you have to have done something in the last few seconds: moved with the keys, jumped, or used an item that is not a kite. Standing still fails. Conveyor belts, hoiks and hovering on a mount also fail, because none of them count as you moving.

A Shadow Candle waives every one of those conditions, which is why any serious farm has one placed or held. Without a candle, holding the attack button down, riding a minecart back and forth between hammered bumpers, or floating in liquid with an inner tube equipped will all keep the check satisfied.

The Rod of Discord is the only reason anyone farms these, and the rate is bad enough to eat entire evenings. Two secret world seeds raise the odds substantially regardless of difficulty, so if you are generating a world specifically to chase one, make that decision before you press create rather than after. Budget for Enchanted Swords in the farm too, since they arrive under the same conditions and travel through solid blocks to reach you.