Amulet Of Held Destiny

Once per day you my roll (4d6) and store that total within the amulet. Before you roll, you may expend a single point of luck to add +4 to this total. Any time you would roll a (d20) you my substitute the stored number instead. If the stored number exceeds 20 the roll will be treated as a 20. Substituting a number causes the stored value to decrease by (1d6).