Cosmic Conversion

   You bathe in the chaotic energy of the Elder Gods, beseeching their favor. When you cast this spell, you can roll a number of your hit dice equal to half your current level. When you do so, your hitpoint total is reduced by the number rolled as your life force is sacrificed to the Great Old Ones. This damage cannot be reduced in any way.

  For the duration of the spell, when you roll damage for a spell, you can increase the Die size by a number of times equal to the number of Hit Dice rolled, Maximum of d20. The spell ends early if your hitpoint maximum is increased during the duration of the spell.

Your hitpoint maximum remains reduced until you take a long rest or are the target of a Remove Curse spell.