Blood Magic (5e Feat)
From D&D Wiki
The skill of blood magic allows you to draw magical power from the life energy that beats within flowing veins, stealing away blood to use for your own ends.
When you finish a long rest, you can expend hit dice to gain one temporary spell slot of 1st, 2nd, or 3rd level. You must already have at least one spell slot of that level or higher from a class feature.
If you have at least one 1st-level spell slot, you can expend one hit die to gain a 1st-level spell slot.
If you have at least one 2nd-level spell slot, you can expend two hit dice to gain a 2nd-level spell slot.
If you have at least one 3rd-level spell slot, you can expend three hit dice to gain a 3rd-level spell slot.
The next time you finish a long rest, you lose any extra spell slots gained in this way.