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Now, I know that healing cantrips are completely broken. So then, my player comes to me with this:

Blood Transfer

Evocation cantrip

Casting Time: 1 action

Range: Touch

Components: S

Duration: Instantaneous

You touch an ally, you sacrifice hit points up to your the amount of your current hit hit points -1. The ally gains temporary hit points equal to the amount of hit points you sacrificed.

I can't tell if this is balanced/broken, because your essentially killing your own hit points to help an ally, is this balanced or broken in anyway?

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Exploitable uses

I'm assuming the intended use of this spell is to sacrifice your own HP to save someone else, and so any use that doesn't result in the caster being depleted of HP is abusive.

Recovering the spent HP

The basic abuse of this spell is to grant someone a temporary hit point (THP) buffer, then use a healing spell or other source of healing to recover the hit points spent. The advantage of this trick over using the healing spell normally is that you can have the extra hit points in advance, preventing instant death from massive damage, lost turns due to unconsciousness, or the problem of the healer having to actually get to you in combat.

It also works with forms of healing that are too slow to be practical in combat, especially short rests and the (already ridiculous) healing spirit spell. The Blood Transfer guy can transfer a large number of HP to someone, splash around in the healing spirit fountain, and (if he's quick) do it again, continue doing this as many times as they want to spend spell slots, then do one more big transfer, take a short rest, and burn hit dice to recover.

Similarly, a druid with Wild Shape can give you the HP of a body he's not even going to keep.

All of these tricks require spending resources to get the HP back--it ain't free. The problem is that all of those resources come back eventually if you survive, and the THP buffer makes it easier to survive anything. If you can convert all your spell slots and hit dice and Wild Shape uses and everything into a giant THP pool, then the solution to most problems becomes "do that, then just beat the monster to death with your face." This is boring, and you shouldn't make the boring tactic the most practical one.

The only way to really fix this is for Blood Transfer to reduce the caster's maximum hit points, like a vampire bite (which is literally a blood transfer, so there's precedent). That sounds harsh, but combined with a reasonably short duration (like "until your next short rest") it really should only block abuses of the spell.

Laundering THP

There are spells like armor of Agathys and false life that give THP with restrictions (self only, short duration). Blood Transfer does not share those restrictions, so it can be used to convert "5 THP to yourself for an hour" to "5 (or more) THP to anyone until their next long rest".

The simple fix for this is to require the spent HP to be real hit points, not THP.

Not quite exploits but discuss with the player anyway

Duration

Blood Transfer doesn't say how long the THP last. By default, this should mean they last until the next long rest (see the last sentence here). It might be smart to say this specifically so the player doesn't think they last forever. (Thanks to Medix2 for pointing this out.)

Disciple of Life and Blessed Healer

Life-domain clerics get various improvements to spells that restore hit points. Granting THP really isn't restoring hit points, so this shouldn't be an issue, but make sure the player knows that before they try to set up some infinite combo around it. (Thanks to RallozarX for pointing this out.)

THP won't actually save you from death.

If you have 0 hit points, receiving temporary hit points doesn't restore you to consciousness or stabilize you.

This is a problem for the intended use of the spell. If this spell doesn't stabilize, I'd say that in advance, or else it's going to be a nasty surprise the first time they try to use it on a dying guy.

(If you fix it so that it does stabilize then it's a straight upgrade of spare the dying. I don't think that's a bad thing--someone might actually use it. But it also significantly changes the aesthetic of the spell, so be cautious.)

This isn't damage.

"Sacrifice hit points" is not language the game normally uses, but you should make it clear that the hit point loss is not damage. It can't be reduced by anything that reduces damage, it doesn't trigger anything that happens when you take damage, you can't be resistant to it, etc. For further protection against shenanigans, I'd specify that the spell only grants THP equal to the actual loss in hit points, not the number you would have lost in some hypothetical case.

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  • \$\begingroup\$ The sleeping thing may not work quite how you think because: "Unless a feature that grants you temporary hit points has a duration, they last until they're depleted or you finish a long rest." Also Blessed Healer states: "When you cast a spell of 1st level or higher..." so it can't apply to cantrips (Removed a bit in the middle since you addressed it perfectly in point 4) \$\endgroup\$ – Medix2 Feb 1 at 2:30
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    \$\begingroup\$ Disciple of Life and Blessed Healer would not interact with this spell as there's no actual Hit Points being restored. Temporary Hit Points are not Hit Points and thus wouldn't trigger those abilities. \$\endgroup\$ – RallozarX Feb 1 at 3:09
  • \$\begingroup\$ @RallozarX Yes, I would agree, but does this player agree? It's worth talking about just to make sure you're all on the same page. \$\endgroup\$ – Mark Wells Feb 1 at 4:08
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    \$\begingroup\$ @Mark Wells It's not about agreeing, it's what it is. "Some spells and special abilities confer temporary hit points to a creature. Temporary hit points aren't actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury." (PHB, p.198) and both of the features mentioned restore hit points specifically. They just don't interact. \$\endgroup\$ – RallozarX Feb 1 at 7:07
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    \$\begingroup\$ Other angle for why Blessed Healer and Disciple of Life don't work: you don't restore THP you grant them. \$\endgroup\$ – Medix2 Feb 1 at 15:36

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