Sacred Seals are unique equippable items that function as an additional passive skill slot. They can replicate many A, B, or C slot skills, or provide unique effects not found elsewhere. You can forge and upgrade Sacred Seals in the 'Sacred Seal Forge' using Sacred Coins and Badges. Upgrading a seal increases its effectiveness (e.g., 'Atk +1' to 'Atk +3'). Since seals are not consumed, you can swap them between heroes freely. They are invaluable for patching up weaknesses, enhancing strengths, or enabling specific strategies that require an extra skill slot. Always keep an eye on new seals released through events like Tempest Trials+.
