TFPH - Ward of Ossein Reinforcement

An armor of overlapping bones forms around you obscuring your form.

Choose any number of d6s up to 5. Roll these d6s; you take necrotic damage equal to the total rolled and your maximum hit points are reduced by the same amount. This damage cannot be reduced in any way. For the duration, your armor class increases by the number of d6s rolled, up to 5.

Your maximum hit points are only restored upon completing a long rest. If the damage from this spell reduces you to 0 hit points you die.