The Fortress of Souls is a boss raid against Soltar, fought across three phases. Each phase is its own fight, and the further you push Soltar's HP down, the better the rewards.

Rewards are threshold-based: you earn the tier for the lowest HP bracket you reach (getting him to 35% pays the 40% tier), not a running total. Every attempt drops Soulbound crystals (Crystal Powder, Crystal Shard, and Crystal) plus Gold, and the amounts climb with each phase.

Those crystals, together with Iron, are spent at the Smithy to forge and enhance Soulbound Weapons. The raid is opened from the Fortress of Souls building.