Eikthyr is the first boss you'll come across in Valheim. Each boss must be summoned at their specific type of Mystical Altar, and the only way to find out the location of such Altars is by interacting with Vegvisir runestones - the small glowing red stones you tend to find near manmade structures on the Valheim map. With Eikthyr, though, you don't have to go searching. Eikthyr's Vegvisir is always at spawn, and forms part of the circle of stones representing the Forsaken at the centre of the world. Just interact with this small red runestone and a marker will be placed on the map over Eikthyr's Mystical Altar.