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 enemies, teleporting around the player slowly, with the caveat that it cannot teleport instantly after being hit. The Occultist will summon several Zombies and Blood Zombies while occasionally summoning Dark Grasps that inflict the Obstructed debuff. If a nearby enemy summoned by the mini-boss has low health, the Occultist will kill it and restore 30 health.