TL;DR: around a 2% increase in attack power per boss/evergaol/POI grace, caps at ten stacks.
When you get invaded by the Fell Omen (aka Morgott) and defeat him, you get a new passive called "Traces of Grace-Given Lord" which says, "Newly found Sites of Grace raise attack power".
I collected the following info from watching a VOD and taking notes. The results:
- Increases attack power by 1.5% - 3% per grace, based on example below. (And based on the assumption that the increase is linear.)
- Caps at 10 graces.
- This only counts sites of grace you reveal, like when you beat a point of interest, a world boss, an evergaol, etc.
- Retroactively counts previously revealed sites of grace.
