I have found the spice a few times, it looks like a brown bag of pixie dust usually in chests but also can buy from merchants. It is more expensive than a essence and is pretty rare.
It does more damage than a fire essence at least it did when I used it, and I think it can be used even though another damage add on has already been used, hazy on that.
Have not found any in this or last playthrough, but I found it like 3 times in the playthrough before that! Random loot strikes again!