The Vortexian is a Hardmode, post-Lunatic Cultistenemy spawned by the Vortex Pillar during the Lunar Events. Their death opens a portal that shoots lightning bolts similar to one of the attacks of the Lunatic Cultist. They are powerful in packs, and can potentially start an overwhelming lightning chain reaction if not dealt with properly.
They seem to resemble the Moon Lord with legs, but without the eyes or the spine. The teardrop design on its chest, combined with the exposed brain further supports this.
The Bestiary entry for the Vortexian: "The cosmic energy surging through these warriors builds up until their untimely demise, resulting in an explosion of lightning."