First thing is your really need to put some kind of limiting factor on the spell casting would make the class really brokenly powerful as there are many ways of getting infinite healing.
Which is what I was worried about.
And if you find a way to limit the healing you would make the class broken in the unplayable sense. The class would also be a bit weak as the character has to spend actions in combat hurting themselves instead of the enemy.
To be fair, they can cast and sacrifice HP at the same time (using the same standard action). They just need to build up for more powerful spells. I do see your point, however.
I think you should just scrap the HP sacrificing mechanism entirely as it has to many problems with integrating it into the game balance. You could replace it with a set amount of blood points per day based on level (like psionics) with the ability to gain a small recharge whenever the character kills a creation with more than one HD (to prevent or at least limit the bag of rats stratagy).
I'll consider removing the HP sacrifice mechanic, though I'm currently working on another idea (more or less a compromise) to balance it. Essentially the Crimson Magus can only spend so much of her life force in one day (the amount is equal to her HP maximum count), but it is still accessed by harming herself. Whenever she casts, she pays the HP cost directly (which also depletes the total amount of life force she can spend). Whenever she chooses to draw life force from taking damage, it goes into her Blood Point pool which she can cast from instead of her HP pool.
Whenever she is healed, her life force isn't. That requires the standard 8 hours of rest to recharge.
Simply:
HP: Standard hit points, must be lost to access life force
Life Force: Equal to Max HP, depleted with casting.
Blood Points: Obtained through the loss of HP (and life force) via attacks, as well as the deaths of other creatures. Can be spent in place of HP/Life Force.
I'll need more clear terms, and I'll need to sort things out a bit better, but maybe that could work? Would it be clearer to say she can't obtain more than her max HP in blood points per day, aside from points she gains from other creatures' deaths?
If that does work, I think I'll bring the price of casting spells back in line with the power point cost for manifesting psionics.
And good point about the bag o' rats. I'll be sure to add that specification in.
Also the class name reminds me of Red Mage from 8 Bit Theater (I believe Crimson Magus is what the character calls his class).
Could be. I haven't read 8 Bit Theater in a long time. I should go back and re-read.
Thanks again.