Potent Life Storage

You have learned how to extract your own living energy and save it for use at a later time. You gain the following benefits:

• Increase your Constitution score by 1, to a maximum of 20.
• When you take a long rest, you gain one Potency die, a d4.
• You may choose to roll your Potency die and add the number rolled to one ability check, attack roll, or saving throw you make. The Potency die must be rolled before the d20.
• As a bonus action, you may choose to roll your Potency die and gain double the number rolled as temporary hit points.
• You can only have one Potency die at a time. A Potency die can not be traded or stolen. Once the Potency die is rolled, it is lost. A Potency die will be lost if you drop to 0 hit points and can not be used for death saving throws.
• If you still have a d4 Potency die when you take a long rest, it is upgraded to a d6.  And so on: a d6 is upgraded to a d8, a d8 is upgraded to a d10, and a d10 is upgraded to a d12.  A d12 Potency die can not be upgraded.
• When rolling for an attack, if the number rolled on a Potency die is greater than or equal to 10 and the attack is also a critical hit, it becomes a potent critical hit.  When you score a potent critical hit, roll all of the attack's damage dice three times.