Cultists Altar is primarily a quest-based landmark 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 around the altars. If a player is to look for such a location, his best chance is to check Ancient Islands.
Cultists Altar appears multiple places throughout the world of Salt but is still an uncommon landmark.The altar 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)
- The Cultists Altar mostly appear on Ancient Islands because the Cult itself is ancient, and therefore uses fitting locations.