Blood Magic

For as long as this mode is active, the caster sacrifices health to power spells instead of expending spells slots, but effects that heal the blood mage are much less effective than normal. 

• Equipping any one of the following items: Blood Ring, Robes of Avernus, Blood Promise, or Sash of Forbidden Secrets, provides the "Improves Blood Magic" effect, which lowers health costs by 25%, resulting in health costs at 60% of original costs. This effect does stack with itself, meaning if all items are acquired and equipped, the HP loss would be at its smallest.
• Healing from Blood Sacrifice and the character's health regeneration are not affected by Blood Magic's healing penalty.
• Blood Magic must be active to cast Blood Sacrifice, Blood Wound, and Blood Control.
• If Blood Magic is deactivated while the caster is using Blood Control, the control is immediately canceled.
• Blood Magic will automatically deactivate when combat ends. 
• If the caster's remaining health is lower than the cost of a spell, casting that spell will kill the caster (or knock them unconscious, rather) when the casting is finished (and the spell will not work). Even if the caster is healed during this casting, as long as the health cost has already been deducted, they will still go down.