Hey again everyone!
One of my gaming friends had this not so shabby idea.
At least on first sight. So it's on you to pick it apart:
"As a *** action you can take a number of CON damage (or spellcasting ability damage) equal to the desired spell level +1. This damage cannot be healed through magic, only through rest."
I think blood magic is great, but I think the ability damage in the proposed mechanic is sufficient that no player would use it (I know I wouldn't) as it can create a significant "death spiral".
I found a third party feat which has a rather elegant manner with which to execute it here.
Alternatively, you could sacrifice hit points (spell level + 1, spell level x 2, or some such) which would make it more likely to be used.
The main idea was not "will be used on a regular basis" but rather as a last resort kind of thing.
Though maybe drop the "+1", so damage is simply equal to the spell level.
I think ability damage is better than hit point damage as hit points are much larger resource and the range between humanoid casters and non-humanoid casters can be way bigger than the the CON difference of them. (Sure larger creatures can still use it more often)
How about negative levels instead?
Anyone else?