Diabolist

About
Diabolists spawn in dungeons after hardmode is activated. They rapidly teleport and fire blazing magic bolts at you that go through walls. The bolts then explode where they hit and create an "Inferno" - a large flaming ball that does massive damage. Diabolists will always teleport to an area that has nothing blocking the path between the Diabolist and you. You an use this to your advantage by creating a narrow area so you know where the Diabolist will teleport to. Diabolists, like other hard-mode caster monsters in the dungeon will teleport on each hit, making them more difficult to dispatch.

Diabolists will spawn when a diabolist banner is present in the dungeon