Overview
Eikthyrnir is a Valheim world location that spawns in the meadows biome, identified internally as Eikthyrnir. World generation places up to 3 per world, with terrain constraints (max distance 1000), and an exclusion radius of 10m, which is why some seeds have far more of them than others. The data tags it as prioritized.
About
Eikthyrnir is Eikthyr's Forsaken Altar in the Meadows. Two Deer trophies placed on the central stone summon Valheim's first boss; up to three of these altars spawn per world.