Life Tap

Either by research of the dark arcana or by a natural talent and mastery over life energy, you know how to tap into your own life force and convert it into magical energy. as a bonus action when you cast an evocation spell, you can draw upon your life force to either cast or enhance the spell instead of using spell slots.

When doing so, you sacrifice hit points equal to one hit die for each spell slot you want to gain, taking damage equal to the amount rolled. you can gain spell slots this way an amount of times equal to half your level. if you do choose to exceed this maximum you receive one level of exhaustion for one spell slot above the maximum. you regain all expanded uses of this ability when finishing a long rest.

For example if a level 7 sorcerer is casting a Fireball at level 3, they can cast it using life tap - receiving 3d6 damage. Alternatively the sorcerer can cast Fireball at 4th level, using a level 3 spell slot and receiving 1d6 damage by using life tap.