Bloody sacrafice.

you  expend any number of your hit dice and roll them.You gain temporary hit points equal to the total rolled.Your armor class increases by 1.Once per round,when you hit with an attack roll,you can deal extra necrotic damage equal to amount of hit dice expended for this spell.The spell ends once you no longer posses any hit points from it.

At higher levels

When cast at 3rd level or above,you roll 1 hit die for each level the used spell slot is above 2nd level,as if you expended said hit die normally.This however does not actually expend them.