Cultists Altar
Cultists Altar is primarily a quest-based location for the last part of the Helping The Innkeeper quest, where the player will encounter Cultists at Night after talking to Lenvell. After this point, the Cultists will start spawning in these locations. If a player is to look for such a location, it's best chance if to check Ancient Islands.
Cultists Altar appears multiple places throughout the world of Salt, but is still a uncommon location.
The location is where a Cultist Ritual will commence as explained in the book "Cultist's Ritual Tome". The Ritual itself will only occur once per map seed/save, as it is only possible to obtain 1 Ancient Skull (Not yet confirmed)
Cultists Altar in sunrise
Source
Trivia
- The Cultists Altar mostly appear on Ancient Islands because the Cult itself is ancient, and therefore uses fitting locations.