Vesper Primer+Constant Effects+Luck of Heroes
Whenever you spend a hero point to reroll a die roll or to grant yourself a bonus before a die roll is made, there is a chance that the hero point is not spent. Whenever you spend a hero point, roll a d20. If the result is greater than 15, the hero point is not spent. You cannot use this Feat when you use the cheat death Hero Point option.