Ancient Altar

The Ancient Altar is a landmark within the vicinity of an Ancient Island that contains a sacrifice altar. Although not a big alter for the likes of human sacrifices, it's meant to be used to sacrifice the Ancient Petrified Heart with the Ancient Ceremonial Dagger in order to summon The Dark Guardian. The dagger is not consumed upon performing the ritual. The Ancient Altar is quite rare compared to the Cultist Altar. Keep in mind that cultists can spawn at those altars, but it seems to be random.

Source

 * Ancient Island

Default seed coordinates

 * 4W / 2S
 * 13W / 22S
 * 31W / 48S
 * 1E / 14S
 * 32E / 30N
 * 34E / 13S