The Occultist is an Pre-Hardmode mini-boss that occurs during a Blood Moon event.
The Occultist spawns naturally during a Blood Moon. This can happen at any time during the event after the Eye of Cthulhu has been defeated. The Occultist cannot be summoned manually using in-game items.
The Occultist behaves similarly to Caster enemy, teleport to a new nearby location after some time with the difference being is that he cannot teleport instantly after being hit. The Occultist will summon Zombies and Blood Zombies simultaneously while occasionally summons the Dark Grasps for up to two or three that inflicts the Obstructed debuff. If enemies summoned by the mini-boss has low health, the Occultist will only kill one enemy at a time and restore 30 health.