The Valkyrie Altar is used to respawn the Valkyrie Queen boss.
Usage
After the Valkyrie Queen is defeated, an Altar appears in the boss room. Offer 10 Victory Medals at the Altar to pay the Queen's toll, then interact with it to summon and re-challenge her. The world difficulty must be Easy or above to start the fight. The Altar is unbreakable and cannot be crafted or collected.
The Valkyrie Altar cannot be crafted. It spawns automatically in the boss room a short time after the Valkyrie Queen is defeated, allowing the fight to be repeated.
