The Seal of Valhalla is a special area introduced in Conquest Mode and serves as the objective of the game mode's The Cursed.
Arguably the design of Gods of Asgard, Seal of Valhalla serves as means to ensure spirits of honorable dead to pass on for Valhalla. However, Valdemar, upon his return as the fallen draug, tampered this sacred area and twisted it so those who died in battle become his undead slaves with aid of his Dark Vaettir bodyguards. Worse still, he also repeatedly conducts a ritual in this area to summon portals from Helheim as means for his undead army to conquer Northgard. As such, it's up to the player and his allies to end his dark reign and ensure honorable deads to go to Valhalla as they destined so.