Blood for Magic

Only the truly desperate would try to sacrifice their very souls to cast a spell.

A path of knowledge that few spellcasters pursue. When out of spells a caster can roll an equal number of their hit dice(ex. a wizard casting fireball at third would take 3d6 damage to cast it), to cast the spell and take the damage as payment for the magic. When the damage from the hit dice reduces the spellcaster to 0, they have the options below. IF the player is out of hit dice. They can also make one attempt to cast a spell with their soul. This is a flip of a coin. Call heads or tails. on a correct calling the spell is cast and the user immediately falls unconscious and gains 3 points of exhaustion. On a failed calling of the coin they proceed with option 2 below.

1. Either the spell misses or is saved against and completely fizzles and the caster still takes the backlash.
   • If this would still kill the caster proceed with option 2.
2. The Spell Crits, in some way. If its not a damage based spell the DM can flavor that how they will. 
   • doing this kills the caster at the end of the spell. Their very essence consumed by the magic.

*If the spell cast is a concentration spell and would kill the character they may on each of their turns after dying roll a concentration saving throw to keep it running.