For this, you will probably want to list the levels.Crimson Razor (Ex)
At 8th level, you gain the ability to unerringly strike at the vitals of your foes. Once per day, you may declare one melee attack a crimson razor. You must do so before you make the attack roll; if the attack misses, you still lose that use. If the attack is successful, than it is automatically a critical hit. You cannot declare an attack both a crimson slash and a crimson razor. You gain an additional daily use of the crimson razor ability at each subsequent level.