The Lunatic Cultist is a Hardmode, post-Golem boss summoned by killing the Cultists that spawn at the Dungeon's entrance after Golem is defeated. Defeating the Lunatic Cultist initiates the Lunar Events.

Why is the lunatic cultist not spawning?

If the Lunatic Devotees are killed and Cultist Archers are lured away and killed far away, the Lunatic Cultist will not spawn. The Cultists will not respawn, either. This can be fixed by reloading the world.

How often do cultists spawn?

They spawn between 22:00 and 6:00, even if the raid has already started. Bug: They may also spawn during daytime on Woods and possibly on the other aforementioned maps, too.