However, it is also important to recognize that Black Spirits are the yin to White Spirit's yang; whereas the white spirits are helpful, the black spirits are dicks. It is also important that black spirits do not give too much without having some sort of drawback for it, and it is fitting for black spirits to screw with the one they're possessing for as much as they're helping them along.
With these two points in mind, here's my idea:
Each Black Spirit will provide a (stacking) boost of +4% to all damage dealt by the user. However, each Black Spirit will come with ONE random penalty, chosen from the following list:
*All damage dealt to the user is increased by (a stacking) +4%
*-1 Rank to a non-Inate skill from the user's equipped classes (each black spirit chooses a different skill)
*Damage received from a randomly chosen element (rare elements are not chosen) will always count as a Weakness, regardless of the user's resistance to that element (each black spirit chooses a different element)