Using hit points to power magic

[Connorsrpg]

Yeah, perhaps this could be done to fuel 'extra' magic. So casting their normal alotment is normal, but to them cast more they must drain hps. I would have preferrred this method instead of at-will cantrips to be honest, but I can see a need for both. Plus it is a good way to get wizards etc to actually use hps in a lot of fights. Also good for when low on spells. (I reckon I might introduce this as a House rule and give it a go).

 

[Vicaring]

I considered this myself, but decided against it because of healing spells; I couldn't figure out how to get them to work in a HP-based system like that.

Instead, I went with a Spell Point variant. Half the normal SP (as described in the DMG), but casters can still cast beyond that. However, every time a caster attempts to cast a spell with no or not enough SP, they have to make a Con Save or become fatigued. I think the DC was 8+SP cost of the spell. What I found with this system was the caster's never seem to have enough SP at low levels, but I was seriously worried about the crazy amounts of SP they get at higher levels.

After that I gave ALL casters d10 HD, but then at every level they have to choose how many of those Hit Points are Spell Points. The decision, once decided, is final. So you could have a first level spellcaster with 8 HP and only 2 SP, or the reverse, a crazy loon with only 2 HP but 8 SP. I like this variant so far, but am unsure how it scales. Again, that Fatigue system for casting without SP is in place.

 

[dmnqwk]

I hope you are considering making anyone who performs an attack suffer the same thing, because why do you feel magic takes more out of someone than swinging a sword in full plate armour?

I think instead of hit points, consider creating a Fatigue system which combines all strenuous activity (fighting, combat, using special abilities etc) as too often people want to pick on spellcasters when spellcasting is already balanced.

 

[bedir than]

If you think that spellcasting is simple and easy, or balanced, a "pay with your blood" system isn't for you. This is a system that makes magic painful, dangerous and possibly evil. It is dark and amazing.

 

[Carcharoth]

For me, the idea conjures unsettling imagery of blood-letting, mutilation, and sacrifice. In the beginning, the carving of runes into flesh, precision cuts across obscure meridians, and the exacting use of various corporal mortification devices. At its height, a mage lies prostrate within a congealing crimson pool, proffering a pound of flesh, blaspheming with what very well might be his last breath.

Man, do I like it.

The idea that a caster is not only capable of creating magic through self-sacrifice but is also willing to do so makes this much more intriguing to me than the type of Blood Magic in which one gains power through the sacrifice of others. Don’t’ get me wrong, I love me some Blood Magic, but gaining power this way is innately evil. With this concept, I can see making a character that surrenders a little of himself each day, and risks the greatest sacrifice of all for the good of others.

I look forward to more.

 

[dmnqwk]

If you are trying to create a fair system when doing this, you should consider removing all saving throws from spells.

If magic requires ritual mutilation and kills so readily, then a lot of first level wizards would be dead, causing them to be much rarer. It's also likely to be illegal and/or outlawed, meaning any casters who cast magic in public are going to be hung like witches in the 17th century. And if magic is rare and wondrous, people wont be capable of developing tactics to avoid it, or resist the effects so allowing saving throws would go against the idea.

So a Hold Person spell suddenly holds for 1 minute, instead of allowing a save each round, making it as powerful as the cost. Or a Fireball dealing 8d6 damage no longer lets you halve the damage (though I would allow Evasion to function as a save for half, fail for full instead of it's current iteration).

 

[steeldragons]

Could it be as easy as just say:

1) Each spell casting costs as many HP as 2 X its spell levels.
2) For each spell you cast that takes you lower than half your HP, you must make a Con. save (DC...13 + spell level??) or lose consciousness for a number of rounds equal to the spell level.
2a) optionally, break it down: 1 round for spell levels 1-3; 1 [10 min] turn per spell level for spell levels 4-6; or 1 hour per spell level 7-9.

i.e. You can continue to cast past your half-HP mark, in emergencies/dramatic moments, to "pull it out of the fire", as it were, but it is not advisable and could be the last thing you do if you don't take out everything/"win" with that spell (getting stabby stabbed or captured while unconscious).

 

[jodyjohnson]

Right, classes with higher hit point totals will be able to cast more under your system and it isn't simply Cleric vs. Wizard. In the PHB, casters are

Bard (Arcane) - D8 hit die
Cleric (Divine) - D8 hit die
Druid (Divine) - D8 hit die
Eldritch Knight (Arcane) - D10 hit die
Paladin (Divine) - D10 hit die
Ranger (Divine) - D10 hit die
Arcane Trickster (Arcane) - D6 hit die
Sorcerer (Arcane) - D6 hit die
Warlock (Arcane) - D8 hit die
Wizard (Arcane) - D6 hit die

This has a very Sword & Sorcery feel and I expect many of the Spellcasting subclasses simply wouldn't be options.

And any of the main full spellcasters would need to rely primarily on weapons and armor early (or those of their minions) but eventually have enough available HP to source some of their magic.

I'd switch the formula to 1d8*Spell Level minus Caster Level and Casting Stat (minimum 0). So at level 1 you are likely spending 1d8-4 (16 stat and level 1) or 0-4hp. And at 17th, 9d8-22 (20 stat and level 17) or 0-50hp or 18hp on average.

I'd leave in spell slots as an upper limit per day.