Found in the Irithyll Dungeon, on a corpse lying in the far-left cell of the second cell block.
For the majority of spells, equipping this ring will reduce its FP consumption by a value between 20% and 25%, while reducing HP by 15%, rounded down. However, wearing this ring does not affect spells which constantly drain one's FP while held, such as Fire Surge and Divine Pillars of Light.
Generally speaking, the higher the base FP cost of a spell is, the greater the FP cost reduction that it receives is likely to be after equipping the ring. For example, the cost reduction of a spell with a low spell cost, such as Deep Soul, will probably be closer to 20% (or even below 20%, in some cases), while that of a spell with high FP consumption, such as Tears of Denial, will likely be closer to 25%. The reduction percentages for each spell are shown below: