Double or nothing

As the name suggests, if an enemy tries to hit you with any sort of attack you can yell “Double or nothing” to cast the spell. After casting roll a D20, 1-10 take double damage, or half damage if you are using it to double damage,  and if using for an item to double you get nothing. 11-20 successfully negate all damage, or double outgoing damage, and double any item you are obtaining.