Diabolist

Diabolists spawn in dungeons after hardmode is activated and Plantera has been defeated. 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, like other hardmode caster monsters in the dungeon, will teleport on each hit, making them more difficult to dispatch.

They are immune to the Poisoned and Venom debuffs but strangely vulnerable to Cursed Flames.

They spawn in dungeons with Tiled Walls.

Tips

 * Diabolists' spells can go through walls, so make sure there are no walls between you and the Diabolist.
 * Since they behave exactly as Necromancers, the best way to defeat both of them is to continuously shoot Shadowbeam Staff beams all around you and move towards them to guarantee that you'll hit them. If you have yet to obtain the Staff, then the best way would be to use any ricocheting weapon, such as Magical Harp or Crystal Storm. Note that Diabolist will teleport away instantly after being hit, making most melee weapons (especially swords) practically useless. If you fill the room with the projectiles of the aforementioned weapons, the Diabolist will certainly get hit, giving him no chance to attack.
 * The Piranha Gun is extremely useful for killing the enemies, as the gun will home in on the Diabolist until it's dead.