Blood Bargain

You can trade your vitality for spell power.

Make the bargain: When you cast a spell or as a bonus action, you may reduce your Constitution score by 2 (to a minimum of 1). For each 2 points reduced this way, you gain one charge, which you must immediately spend to do one of the following (you can’t bank charges):

• Create one 1st-level spell slot, or
• Increase the level of a spell slot you’re about to expend by +1 (you can’t exceed the highest level you can normally cast), or
• (If you have Font of Magic) gain 2 sorcery points.
  
  You can combine charges to form higher-level slots: 2 = 2nd, 3 = 3rd, 4 = 4th, 5 = 5th.

Limit: You can activate Blood Bargain a number of times per long rest equal to your proficiency bonus. You can’t use this feature if it would reduce your Constitution to 0 (you die at 0).

Consequences: All normal effects of a lower Constitution apply immediately (hit point maximum, Constitution saving throws, concentration checks, etc.). Reductions from multiple uses stack.

Recovery: At the end of each long rest, restore 1 point of Constitution lost to this feature. Magic that ends ability-score reductions (such as greater restoration) restores 2 Constitution lost to this feature per casting. Regenerate restores 4, and wish restores all. Lesser restoration restores 1 Constiturion lost to this feature per casting.

Variant (Spell Points): If your table uses the DMG spell-point option, each charge grants 2 spell points instead of a slot/level increase.

Notes:
Also called “Magical Constitution” at some tables.
Intended for classes with Pact Magic or Spellcasting Class feature