Found in Lothric Castle, just before the Dragonslayer Armour. From the Dragon Barracks, head across the bridge and through the entrance of the castle, heading upstairs and to the left. Continue forward and outside, where an entrance to the right should be followed, into a small chapel with the shortcut lift to the Grand Archives. Head to the left and outside onto a balcony where a corpse can be found holding the ring.
Boosts attack by 20% when HP is below 20%.