Vortexian

Vortexians belong to the pillars that drop into the world once the Lunatic Cultist is dead. There is no summoning item and no way to meet one early. Beat the cultist at the ritual outside the dungeon, wait for the lunar event to begin, then travel to the Vortex Pillar and they will be part of the guard walking the ground under its shield. They only spawn inside that pillar's territory, and once the pillar falls they stop coming, so a banner farm means beating the cultist all over again.

The corpse is the dangerous part. Every Vortexian that dies opens a portal near you, and about a second and a half later that portal throws a lightning bolt. The bolt locks onto where you were standing at the instant the portal appeared, not where you are when it fires, and a glowing line draws the path in the gap between. Keep drifting sideways after each kill and the bolt lands in empty air behind you. Stop to line up your next shot and it lands in you. The portal itself does no contact damage, so the beam is the only thing to respect.

They cover ground on foot faster than most players sprint, so backing away along flat terrain does not open the gap you want. The real problem is what happens to a group. Wipe out a cluster with one wide hit and you get a cluster of portals, each aimed independently, and the two or three lines you did not notice are the ones that connect. Chip at the edges of a crowd instead of detonating the middle of it, and never reposition immediately after a multi kill. A chain of these can flatten a fully geared character in a couple of seconds.

Portals are picky about where they will appear. They need open air above and slightly off to one side of you, a clear line of sight, and no packed solid material crowding the spot. If nothing in range qualifies, no portal spawns at all and the kill is free. Fighting under a low ceiling of real blocks removes most of the eligible space. Platforms and planter boxes do not count as obstruction, so the usual platform arena will not protect you here.