Now, I don't think I have posted this before but I was thinking about it again today and thought I should share it for all those DM's out there.

I have a hidden Blood Magic mechanic I use in all of my games but don't tell my players about. They usually discover it in a moment of desperation when they are out of spells or want to cast something at a higher level. Some of my friends know about it and are wary to use it. To date, it has killed 50% (4/8) of the characters who have taken advantage of it. So I personally don't think it is terribly game-breaking. It is also a forbidden technique in my fantasy worlds and thus can lead to narrative trouble later. (Also, bad guys can use it which can be so much fun. A final spell cast in desperation.)

The Rules:
A magic-user can cast a spell without a spell slot by substituting their own lifeforce in the casting.
When a character uses Blood Magic they must first sacrifice a number of hit points equal(=) to 1 caster class hit dice plus(+) 1 caster class hit dice per spell level.
For example, if a Wizard Casts a 3rd level Fireball using Blood Magic. They must first spend 1d6 + (1d6 x 3 (spell level)) and take that total as True Damage (no resistance/immunity).
If the caster survives the spell is cast as normal.
If the caster dies while casting Blood Magic it is an automatic Critical Hit. Alternatively, if the spell has a saving throw it cannot be saved against.
If the caster does not have enough hit dice to spend on their spell the damage is still taken but the spell fails to cast. If the caster dies in this way the spell still fails to cast.
Additionally, those hit dice that are used in the casting are considered used up. For the example above, the Wizard has used up 4 hit dice and cannot use them for recovery during a short rest. The hit dice recover as per the normal rules.

I have yet to have a caster who knows about this reach level 20. Where I am aware they would basically gain an additional 2 lvl9 spell slots. But level 20 is chaos anyways. Plus the need to use it at 20 plus the damage the player would take would likely be their end anyways.

Enjoy.

This feels like something that would be better integrated into a subclass or cursed magic item rather than a universal feature. Especially Sorcerers, since the theme of their magic comes from their bloodline more often than not anyway.

Also, definitely something that would be better off in the Homebrew section of the forums, since you especially call it homebrew in the title. 

I did try something like this as well, never got a chance to finalize it but I do think I got it sorted out how to use blood as a power source.  It was def more of subclass idea and required an attack/ritual to gain pips similar to sorcerer points.  I should finish it someday and try to test it.

good luck with your idea!

Have you seen the Krynn reworked Magic Adept UA?  There's a hit die expenditure option for the Black Robes, not as drastic as yours, but it's something WotC is playing around with as they design the Dragonlance book.