Necromancer

They appear in the dungeon and only after Plantera is dead, alongside the rest of that roster. Nothing summons them and there is no event attached: once the gate is open they are part of ordinary dungeon spawning. They strongly favor the brick walled sections, so a room built out of anything else sees far fewer of them, which is worth knowing when you pick where to farm.

There is no projectile to dodge. The shot resolves instantly, so the only thing you can react to is the windup before it, and the only reliable dodge is not standing on the line when it lands. The beams bounce off surfaces and keep traveling a long way afterward, so a shot that missed can come back around a corner into you. It is not aiming those bounces deliberately. It shoots at you and the ricochets are incidental. What actually hurts is the Slow it stacks on, because everything else down there already moves faster than you do, and a Diabolist fireball is much harder to leave behind while you are crawling.

It relocates every few shots and has an even chance of blinking away the moment you hit it, which makes slow single-shot weapons miserable to use against it. Two things work. Either kill it in one burst before it can move, or use something that follows it. The Piranha Gun latches on and keeps chewing through walls after it vanishes, and a Blade Staff does the same through minions, often bouncing it around so constantly that it never lands a shot.

Its teleport is picky. It needs a solid floor with unsafe dungeon wall directly above it and clear headroom, and it will not appear right on top of you. If it cannot find a legal destination it simply stays put and restarts its cycle. That is exploitable. Fight one inside a small pocket you have floored and walled yourself and it runs out of places to go, which turns the most irritating enemy in the dungeon into a stationary target.