Blood Casting

General Feat (Prerequisite: Level 4+, can cast non-cantrip spells)

You gain the following benefits:

Ability Score Increase. Increase your Constitution score by 1, to a maximum of 20.

Blood Casting. You gain the ability to cast spells using your life force as fuel. Whenever you cast a spell of 5th level or lower, you can choose to cast it using hit points instead of expending a spell slot. If you use hit points, you take damage based on your class’s hit dice according to the table below (if you have multiple hit dice, use the highest one), and your Hit Point maximum decreases by an amount equal to the damage taken (until a Long Rest). This damage cannot be reduced in any way, and it ignores Temporary Hit Points. 

Taking this damage creates a visible wound or injury somewhere on your body. You choose where this injury appears and what it looks like.

If this damage causes you to drop to 0 HP, the spell still takes effect. You cannot use blood casting to cast spells at a higher level then your highest level spell slot, but you can upcast spells using this feat.