Blood Mage (5e Feat)
From D&D Wiki
Prerequisites: The ability to cast spells, Constitution 13 or higher
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. In addition, the blood in your body is supernaturally under your control, and cannot be taken from your body unless you allow it, even while incapacitated.
As a bonus action, you can expend a certain amount of hit dice to gain a temporary spell slot. You may not have more than one temporary spell slot at any time and any new temporary spell slots replace existing temporary spell slots.
If you have at least one 1st-level spell slot, you can expend two hit die to gain a 1st-level spell slot.
If you have at least one 2nd-level spell slot, you can expend three hit dice to gain a 2nd-level spell slot.
If you have at least one 3rd-level spell slot, you can expend five hit dice to gain a 3rd-level spell slot.
If you have at least one 4th-level spell slot, you can expend six hit dice to gain a 4th-level spell slot.
If you have at least one 5th-level spell slot, you can expend seven hit dice to gain a 5th-level spell slot.
At the end of a long rest, you lose any additional spell slots created this way.