How to Summon Eikthyr in Valheim
Eikthyr won't just appear in-game uninvited. That's actually quite considerate when you think about it, since surviving can be tough enough in Valheim without a giant mystical stag demanding you fight him to the death if you step into the wrong part of the map.

Altar
Eikthyr's Forsaken altar is located in the Meadows. To the left of his Sacrificial Stone is a small glowing Runestone called a Vegvisir; interacting with it will add his closest summoning location on the map.

Gather Deer Trophies
Eikthyr requires an offering before he'll grace you with his presence. You need to place 2 Deer trophies on his Forsaken altar. To obtain these trophies, you'll need to hunt and kill deer in the Meadows - the starting biome of Valheim. It's not a guaranteed drop, so you may have to hunt quite a few deer to collect the requisite trophies. Elite Deer (with 1 or 2 stars next to their Health Bar) have a higher chance of dropping Deer Trophies.

Offer the Trophies
Once you have 2 Deer Trophies, place them on the Mystical Altar to summon Eikthyr. After a few seconds, Eikthyr will spawn, and the boss fight will begin.

Defeating Eikthyr
The chained stag is your first real battle in Valheim. Taking it down isn’t much of a challenge. It just requires patience and preparation.
Before taking it on, be sure to craft available armor, a weapon, and a shield; cook meals to maximize health points.

Conclusion
Summoning and defeating Eikthyr in Valheim is a manageable task with the right strategy. By locating Eikthyr's altar, collecting the necessary Deer Trophies, and properly equipping yourself, you'll be ready to take on this formidable boss and prove your dominance in the world of Valheim.
