Cool idea!
You should make the dark prophet that deals 10 damage to the player that lost the coin toss.
On the balancing side: i would make this one-shot ability, like deja-vu and hatch. You activate it, you lose it.
I would also lower the damage and the cost of the creature.
I would make the cost 3
![Light :light](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/light18x18.png)
|3
![Light :light](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/light18x18.png)
and the ability:
![Light :light](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/light18x18.png)
Prophecy: deal 5 spell damage to starting player (one use).
Since this can be used with a reflecting shield, you can use it even if you won the coin toss.
Thinking again, since this damage is driven by luck and odd, entropy could fit better than light, but i don't know, a light prophet and a dark prophet seems like a good idea.
How about Minor False Prophet | False Prophet for the dark version?