Diabolists attack by firing blazing magic bolts that can travel through blocks at the player. Their magic bolts explode, creating an "inferno", a large flaming sphere that lasts for nearly 4 seconds and deals massive damage on contact, inflicting the On Fire!debuff. Diabolists teleport to a different location on-screen every time they are hit.
Diabolists can only teleport onto solid blocks. If there is no valid place to teleport to, they will remain in the same location.
They are only able to teleport while resting on a solid block.
In Expert Mode, Diabolists have a 0.13*1/800 (0.13%) chance to drop two Inferno Forks at once. The chance to drop exactly one Inferno Fork is 7.25*29/400 (7.25%), hence the chance to drop at least one is 7.38*59/800 (7.38%).
Since a Diabolist's shots go through solid blocks, taking cover is usually not an option.
Weapons that stick to enemies are exceptionally useful, as they remain on the Diabolist after the teleport. The Piranha Gun, Daybreak, and Bone Javelins are viable options.
Weapons that deal high damage in single hits are effective against Diabolists. Weapons that fire multiple projectiles simultaneously may be able to land multiple hits before they teleport as well.
Weapons with homing capacities or minions can continue to track Diabolists after they teleport.
Like other caster AI, Diabolists cannot teleport to any space less than 4 blocks or more than 20 blocks from your feet, will not teleport to blocks covered in lava, requires at least 2 solid blocks(or platforms) and 2x3 clear space to teleport to, and it will not attack if constantly damaged. These facts together can be used to set up a safe dungeon farm if the only space they can spawn in/teleport to has a pressure plate with spiky ball traps.
The Bestiary entry for the Diabolist: "The undead who bear the Diabolic Sigil wield flames as intense as any in the underworld, consuming all in a scorching inferno."