Seals are tied to your parry button. Each one changes what that button does, so check them before you commit. The Infinite Seal is a timed Parry. The Untarnished Seal is a Guard that can Perfect Guard. Vatra's Seal Hardens your body. The Slayer Seal turns sidearms into Break tools and adds a charged punch, but it invalidates achievements, so do not use it if you want them.
Seals Overview
All Seals
This seal is no doubt grand, but unfathomable.
The Gloomslayer once wielded this seal on a doomed crusade.
Both symbol and weapon, the seal has been restored to its former glory.
Used in stonebirth rituals, this seal possesses a fraction of its creator’s power.