Found in the Untended Graves.
From the shrine's entrance, follow the path on the right. The ring can be found near the same grave where Hawkwood usually goes to pray in the Cemetery of Ash. It is guarded by a greataxe-wielding Black Knight and will appear only after the Abyss Watchers have been defeated.
Boosts critical damage by 30%. Additionally, attacks like backstabs and ripostes may have slightly different and faster animations, depending on the enemy, which may knock them down in different ways and deal even more damage than the flat boost should add.