All-in Coin

A completely clear crystal coin that the result is unknown until you make an attack.  Flip the coin on an attack.  If the result is correctly called, the attacks automatically hits with double max damage.  On failure, you roll damage against yourself.  Can be used with any attack including spells.  If used for healing, double max healing points on success. On a failure roll damage instead healing.  After coin toss, but before the result is known, the coin can be thrown at a target to pass the result to friend or foe.

Notes: Short rest.