Mischief

When this enters the battlefield, add {b}.
every turn, check if your devotion to black is more than seven. if true, add {b}{b}{b}{e}{e}{e}.
pay {e}{e}{e}{e} with a devotion to {b}: tap all creatures but this and all tapped creatures get -1/-3.
{4}: Level up. (this starts at level 1. level up to get +1/+1, and also decrease all ability costs by 1.)