When compared to Double Strike and Crushing Blow(two talents on the warrior path):
1. It is too weak (50% chance to hit)
2. It is too similar to above mentioned talents(does double damage)
50 % chance to cause a critical attack and 50% chance to damage the hero casting it using his normal attack value, self damage should never be critical.
Why would this idea work:
1.Assassins are a good deal about critical damage
2.Their normal attack should be lower than the warrior's so they would not suffer as much when the attempt fails.
3.It would be spectacular when the hero pulls it off.
Balance could be achieved by increasing the recharge timer of the ability.