CONTAINS SPOILERS

First, make sure you mark the location of Eikthyr on your map by using the glowing red rock at your original spawn location (where you start the game). Make your way to his location. The clue "HUNT HIS KIN" is revealed if you use the red stone in front of the altar. Offer two deer trophies (random drops by hunting deer) at the Mystical altar to summon Eikthyr, the first boss.