Blood Magic

You have learned to sacrifice your own flesh and blood to fuel magic beyond what you would be capable of casting. As an action, you can expend a hit die (dice) and regain a spell slot whose level is equal to the number of hit dice spent. The spell slot regained can not exceed the max spell level you can cast at your class level. 9th level spell slots can not be regained through this feat. 

NOTE: Warlocks must expend hit dice equal to the spell level at which their spells are being cast. (ie a 6th level Warlock’s two spell slots are cast at 3rd spell level. The warlock with the Blood Magic feat. at this level would need to expend 3 hit dice to regain one slot.)