Thanatopic Seal

As you cast this spell you take 16 damage. The spell has a reservoir that can hold a number of hit points equal to your maximum hit points. When you cast this spell there are 8 points in the reservoir. At the end of each day you may roll any number of hit dice and take damage equal to the result, then add that many points to the reservoir. The first time you would be reduced to zero hit points while this spell is active, you gain hit points equal to the points in your reservoir. The spell then ends.