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I'm in the midst of creating a magic item that would steal hit points from enemies and store them. Once a certain level of hit points have been stored, the user of the item would be able to convert them into a spell slot. However, I'm not sure what the 'exchange rate' of hit points to spell level should be.

Here's my idea so far (All the numbers below are just spit-balled.):

Staff of Life Stealing - On a succesful melee attack, the staff deals 1d10 hit points worth of damage. Those hit points are stored within the staff. As a standard action, the user can convert those hit points to spell slots at the following rate:

  • 5 hit points = First-level spell slot
  • 15 hit points = Second-level spell slot
  • Etc...

Are there any rules in D&D 3.5 that provide a guideline for exchanging hit points for spell levels? I'm open to looking at feats, spells, prestige classes, monster abilities, etc. All of those are fair game. The specific source doesn't matter, so content from Dragon Magazine and other OGL sources is fine A correct answer would say "Such-and-such ability equates 10 hit points to a first-level spell slot."

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No existing item, feat, or class feature allows you to do this. Restoring HP infinitely out-of-combat is an easy trick to accomplish, which turns this into infinite spell slots if you have the time. And it’s pretty easy to make it not take very long at all.

In other words, this is not a balanced idea.

Best case scenario: no one goes out of their way to abuse this

The closest comparison available is a pearl of power; this replaces a 1/day limit with an HP cost. Well, what does it cost to restore HP?

A wand of lesser vigor costs 750 gp, has 50 charges, and regenerates 11 HP per charge. That's 550 HP for 750 gp. This is generally considered the most efficient HP-for-gold option in the game that can be used by just anyone (with UMD). That comes out to 1.364 gp per HP.

A pearl of power costs (spell level)2×1 000 gp. But a pearl of power is a 1/day item – a 50-charge item costs, by the guidelines,1 2½× more than a 1/day item. So by that metric, a spell slot costs (spell level)2×3 410 HP. Which is a ridiculous number, but it’s the closest thing to guidance that the existing rules provide. If you go below this number, the staff turns wands of lesser vigor into cheaper pearls of power.

The fact that it takes a lot of time definitely justifies a price reduction. Assuming you have all the wands you could want, and ten willing targets, you are adding 100 HP to your total each minute. 30 minutes for a first-level spell slot is bad; the 24+ hours for a 7th-level spell slot is completely pointless.

But when we are starting at the spell level squared multiplied by 3 000 HP, even if you drop it to 1% of that number you’re still talking about (spell level)2×(30 HP), which is dramatically higher than any of the other suggestions here. And it’s really hard to justify dropping the cost of pearls of power down to 1% of their cost.

1 Specifically:

Charged (50 charges): ½ unlimited use base price

Charges per day: Divide by (5 divided by charges per day)

For 1/day, that’s ⅕, versus the 50-charge’s ½. ½/⅕ is 2½.

More realistic: someone gets Fast Healing

As soon as someone gets Fast Healing (or Regeneration), this staff turns whacking them into free spell slots. Dragon shamans and dragonfire adepts can do it at level 1 (though they can only restore up to 50% of their HP; just have the cleric heal them back to 100% with all the free spell slots they can get). Various other options for it are available. Goodbye, one of the few somewhat-relevant limitations on spellcasters.

Conclusion: This is not a good idea.

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  • \$\begingroup\$ Would changing the damage the staff does to be Ability Damage rather than hit point damage help the issue at all? (It wouldn't stay 1d10 damage.) Ability Damage seems like it's less trivial to heal. \$\endgroup\$ – Discord Dec 18 '14 at 17:56
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    \$\begingroup\$ @Discord Ability damage would make the weapon more likely to actually be used offensively (as is, no one should ever actually attack an enemy with the staff), and would somewhat limit the abuse possible. But there are a lot of rather-efficient ways to heal ability damage too, including a version of Fast Healing but for ability damage (specifically, Naberius the Grinning Hound vestige in Tome of Magic). So ability damage helps, but not enough. \$\endgroup\$ – KRyan Dec 18 '14 at 18:00
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    \$\begingroup\$ @Discord On the other hand, Ability Burn might be more appropriate – no one is going to be willing to let you burn their ability scores. But being able to offensively use Ability Burn makes it a rather-potent weapon; spell slots on top of that is just phenomenal. \$\endgroup\$ – KRyan Dec 18 '14 at 23:34
  • \$\begingroup\$ A spell slot does not cost the price of a pearl of power. It costs the price of a single use, use-activated item, such as a potion, which is spell level × caster level × 50 gp d20srd.org/srd/magicItems/creatingMagicItems.htm \$\endgroup\$ – Jean-Luc Nacif Coelho Sep 30 '16 at 0:59
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On the actual question:

Are there any rules in D&D 3.5 that provide a guideline for exchanging hit points for spell levels?

Answer: Not to my knowledge.

And now a tangent (that includes a real suggestion).

To be blunt, there is probably no such guideline because creating a general mechanism to turn hit points into spell slots is a bad idea!

The reason it's a bad idea? Hit points are trivial to restore out-of-combat (consider creatures with Fast Healing, for instance), implementing an item like this would make spell slots similarly cheap.

This doesn't mean items that involve trading HP for spell slots is necessarily a bad idea. It just means you'd have to balance the item with additional, more traditional restrictions - meaning cost, activation actions, and most importantly, uses per day.

Personally, I recommend viewing the item as a Pearl of Power with a flavourful twist.

For an example of an item that turns HP into spell slots, see the Blood Claw Choker from the Magic Item Compendium (page 203). The conversion is at a rate of 2HP/spell level - but note the restrictions on its use.

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Cure

Let's calculate minimum amount needed to prevent this staff from being "cure an army for free" item. Obviously, it must cost more than Cure spells it can buy.

  • Cure light wounds: 1st level, 1d8 + cl HP ( 4.5 + 5 = 9.5 average, 13 max )
  • Cure Moderate Wounds: 2nd level, 2d8 + cl HP ( 9 + 10 = 19 average, 26 max )
  • Cure Serious Wounds: 3rd level, 3d8 + cl HP ( 13.5 + 15 = 28.5 average, 39 max )
  • Cure Critical Wounds: 4th level, 4d8 + cl HP (18 + 20 = 38 average, 52 max )

Counted at maximum caster level, because that's when abuse would be potentially most serious. You should calculate HP prices to be higher than average what one can possibly gain. How much higher, is your call.

tl;dr At least about 10 HP per spell level if you want to make sure nobody will use it to routinely generate slots and HP for free. At least ~ 13 HP / level if you want to make sure total HP of someone "looping" cure with your staff can't generate HP for free even on luckiest rolls. But it'll work only at low levels.

Higher levels - Heal

Heal can cure up to 150 HP at 6th level. This gives 25 HP / spell level.

Mass Heal can heal 250 HP at 9th level. Multiplied by the number of creatures you can cramp there, but 20 is totally reasonable. This gives about 600 HP per spell level. Possibly more.

I ran regression on cure and heal series, and it gave me this equation:

HP = 9*​e( spell level *​0.47)

That seems to work, but admittedly it is overcomplicated.

Spellfire

From Forgotten Realms setting.

A spellfire wielder can ready an action to absorb spells targeted at her as if she were a rod of absorption. She gets one level of spellfire energy for every spell level absorbed.

 

A spellfire wielder can also heal a target by touch, restoring 2 hit points per spellfire energy level expended for this purpose.

But this exchange rate is for the exactly opposite what you are trying to do, so I believe it shouldn't be applied.

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  • \$\begingroup\$ The cure spells are notoriously inefficient; they are not a great basis for comparison. \$\endgroup\$ – KRyan Dec 18 '14 at 16:09
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    \$\begingroup\$ @KRyan inefficient in comparison to? \$\endgroup\$ – Mołot Dec 18 '14 at 16:11
  • \$\begingroup\$ A useful and meaningful way to spend your actions? Or, for example, heal -- as a 6th-level spell, under your toughest suggestion it would cost 78 HP, and then heal a minimum of 110 HP (and provide a powerful restoration-type effect). You need a minimum of 20 HP/spell level for that. And mass heal would then cost 180 HP, but heal several creatures for a minimum of 170 HP. At CL 25, mass heal heals 250 to multiple creatures, meaning you're going to need a massive HP/spell level ratio to cover. \$\endgroup\$ – KRyan Dec 18 '14 at 16:15
  • \$\begingroup\$ @KRyan ok, updated \$\endgroup\$ – Mołot Dec 18 '14 at 16:16
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I found an addition to 3rd edition that has the spell "Spell Restore".

The source is From the Laboratory of the Mad Wizard Shadmar by Nighthawk Interactive Games On page 16:

Spell Restore
Transmutation
Level: Sor/Wiz 8
[...]The caster gains 1d2 spells per day for 1st, 2nd, 3rd, 4th, and 5th level spells. [...]The payment for these spells is in the form of 3 hit points per spell level for each spell slot regained.[...]

I didn't want to put the entire text here since this is copyrighted material, but this page is available on google books here

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There are relatively few abilities that convert HP into spell slots, but there are a great many that convert spell slots into HP. A reasonable way to proceed might be to ask yourself "how much less efficient should this HP -> spells process be, compared to the reverse process, a.k.a healing?"

The basic 'cure wounds' spells are d8 per spell level (1d8 for Cure Light up to 4d8 from Cure Critical), plus caster level. Assuming a caster level of the lowest possible to cast the spell (and no other bonuses), that's an average of:

Spell Level    Character Level  Avg. HP Healed
1              1                5.5
2              3                12.5
3              5                19.5
4              7                26.5

I don't know whether you plan this item to be usable for divine spells, but clearly it should cost more HP to gain a spell slot than you can feasibly use that slot to heal, otherwise you're simply encouraging the party cleric to hit himself (or the fighter. I mean, that's what they're for, right?) with the staff repeatedly for infinite spells.

Now, this is obviously an incomplete answer - the exact amount of relative efficiency is something you'd still have to determine. I'll edit it if I come across more stuff that seems relevant.

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  • \$\begingroup\$ The cure spells are notoriously inefficient; they are not a great basis for comparison. \$\endgroup\$ – KRyan Dec 18 '14 at 16:08

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