For each creature target opponent controls, tap it and roll a 6-sided die:
On 1 or 2, that creature deals damage equal to its power to its controller.
On 3 or 4, that creature deals damage equal to its power to itself.
On 5 or 6, that creature deals damage equal to its power to you.
I know that Magic doesn't currently expect you to have a 6-sider to play the game, but personally, I think maybe that should change to open up a little more design space for future cards, and expecting the player to have dice around is really useful anyway, for planeswalkers and counters and such. This could be easily altered to flip a coin instead (remove the 3-4 result and lower casting cost of the spell).