This looks interesting.
It made me think of 2 variants that takes the idea further
Name: ___
Creature
Cost: 8

? [somewhere in the 5-9

range]
Stats: M|4
Effect: Enemy creatures suffer -1|0. ___'s attack equals the number of enemy creatures.
Notes: Attack can go negative.
Name: ___
Creature
Cost: 6

? [somewhere in the 4-7

range]
Stats: 3|M
Effect: Enemy creatures suffer 0|-1|0. ___'s hp equals 15 + the number of enemy creatures.
Notes: This can kill 1hp creatures but ___'s hp will not increase.
I do not have time to pursue these ideas so anyone may "steal" them if they want.