[Miller]

Attune Grudge doesn't seem to be increasing the damage bonus from Black Spirits at all.

I've tested on the test server and saw no difference between before and after reroll. I also made sure I didn't roll attune grudge again.

[Neus]

Posting how and on who you tested this with is important, because it may reveal a problem elsewhere. For example, I'm assuming you tested it on a Chimera and not a natural member of the races you had black spirits with, which would result in a slightly different bug than what is being described.

[Miller]

My bad, I was testing it on an Omina possessed with chimeric bands.




They were both possessed and had chimeric bands on which treated them as the 'Beast' race.

The damage didn't change after rerolling so it's likely that it's either Beast or the player not counting as the Possessed race when using spirits.

I'll be testing more and will update with any extra findings.

Update:
Removing all of my black spirits resulted in no damage loss whatsoever.
Weapon used was a quickdraw that did roughly 26 damage per hit against a possessed Omina wearing Chimeric Bands and that remained static with or without attune grudge and black spirits.

[Neus]

To be clear, I made a change regarding this that likely fixes it, but you'll have to test it again on the testing server when that happens.