more permanents = more damage

[Mort]

This is a very rough idea, but I thought it would be cool to have a card that deals more damage the more permanents your opponent has. This could be a spell, a creature, or even a weapon.
I was thinking of "deals X damage. X is the number of permanents under your opponent's control"
This should probably be a gravity card.
Also, I have no idea how much this would cost :/

Any suggestions/improvements/critisism would be appreciated.

[storyteller]

Gravity Goblin | Gravity Gremlin
5 
2|5
  deal damage to opponent equal to the number of permanents they control

[NonexistentFlower]

Collapse | Destroy
5 | 4
Deal damage to target creature or player equal to the amount of permanents its owner or they control.
Might need rewording and costfixing.