Direct answer
You have found the summoning place of one of the Forsaken. Make the correct offering at their altar and they will come. Be wary though, the Forsaken are not easy prey, so craft a wicked weapon, don your finest armour, and eat a hearty meal before engaging them in combat.
Eikthyr · Getting there

Result
Go to the location: Eikthyr → Next action: Mystical Altar
Game homeValheimAll games