What level and class should have access to this homebrew ability-swap spell?

Question

A friend of mine sent me this spell for Dungeons & Dragons 5e from somewhere:

Swap

3rd level transmutation

Casting Time: 1 action
Range: 30 feet
Components: V, S, M*
Duration: Concentration up to 1 minute

Choose a creature that you can see within range. Also choose one ability when you cast the spell. Until the spell ends, the creature and you swap that ability score. If the creature is unwilling, it can make a saving throw to avoid, the spell. The saving throw is determined by the ability score you are targeting.

* a small reflective object, like a mirror

I actually really like the idea of this spell, but no doubt a spell that has the listed effect above is likely of a higher level than listed, as it can essentially reduce the CR of a creature/encounter greatly via new stat penalties. More importantly, the spell has no classes set as of right now.

The only edit that is being added to the listed spell is: "The target may attempt another saving throw at the end of each their turns, ending the spell on a success."

With that in mind, I was looking for input on:

• What level would be proper for this spell's power?
• What classes can take this as a spell? (I was thinking Wizard, Cleric, and Bard)

Answer 1

In my opinion, the spell as written is very unbalanced and in perfect position to be abused by a wily player, instead of just pointing out what is wrong with this piece of homebrew, I'll step by step show you how to improve it, and balance it:

Swap.

4th level transmutation

Raised to a 4th level spell due to the overall balance of the spell; however, this can be changed if someone with more knowledge about spell balance says otherwise.

Casting Time: 1 action
Range: 30 feet
Components: V, S, M (a reflective surface, like a mirror)

The materials should be defined within the actual "Material" component section of the components. It also only acts to disadvantage players if you limit it to a "small reflective object” - and you will end up debating with players about what qualifies. It's much easier to just let it be a reflective surface. That way you could have a creature in front of an actual full length mirror be affected, as long as it's within reach of the player.

Duration: Concentration, up to 1 minute

Choose one creature of your choice that you can see within range with a CR less than the level the spell is cast and choose an Ability Score you wish to swap with them. An unwilling creature that succeeds on a Wisdom saving throw is unaffected.

Rephrasing this section to be more in line with spells that already exist like levitate that have Willing/Unwilling saves. Added the bit about CR to balance it, and add a global cap at CR 9 so that you will not get the option to swap scores with any ungodly high CR creatures or ones that by default have scores higher than that of a player. Restricted the saving throw of the spell to Wisdom because that is the score most associated with resisting the effects of transmutation. This can easily be swapped with Intelligence or Charisma but I don't advise letting it be any "martial scores" (Strength, Dexterity, Constitution).

For the duration of the spell's effect, the caster swaps the chosen ability score with the willing or unwilling creature for their own. You cannot swap a swapped score with any other creatures or casters with this spell. Once the spell ends via any means, the effect is reversed and both the caster and targeted creature return their ability scores to the state they were in before the spell had been cast.

Added the clause about swapping swapped scores to please the dark wanderer as this is a very legitimate and abuse-able concern. Tried to add a clause regarding the clarification of the spell as being temporary, however it might not be worded optimally, feel free to remove that bit if your players are understanding of how the spell is intended to work.

• a small reflective object, like a mirror

This note can now be removed as we declared the materials higher up in the spell.

I believe this spell should only be available for Wizards, Sorcerers, and the subclasses that gain access to their spell lists.

Before I can say whether this is actually properly balanced or not, I'd have to play-test it with my group, but for now I think this is a little bit of a more balanced version of what you posted, while retaining the spell's intentions.

Answer 2

Exactly as written, this spell should be no lower than 6th level. Mucking about with ability scores is too powerful to be anywhere beneath the level 11 one-class barrier.

Also exactly as written, a good start would be making this spell available to the same classes that get Feeblemind (Bard, Druid, Wizard, Warlock).
Feeblemind is a good comparison tool as the only other spell that mucks with ability scores.

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That said, this is a bad spell. It's too generically powerful, and too generically versatile.

Offensively:
It's got the optimized capability of applying a -15 to all checks, saves, and attack rolls with a particular ability score.
It can easily apply the equivalent of Disadvantage (-5).
It functionally applies disadvantage (or worse!) to the additional saves to end the effect - as they would use the new ability score.
You can also use class features to ensure failure, granting yourself (and your party) a full minute to hit them with near-guaranteed powerful spells that use the same saving throw.

Defensively:
Party member got feebleminded? Give them a +5 instead of a -5 on their next save against it.
Or really any save/check that you know is going to happen, just shove the highest modifier into the relevant character.
And of course, you have to be cognizant of shenanigans with temporary ability score boosts, and shenanigans like a simulacrum of a creature currently affected by this spell...

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What follows is balancing recommendations:

Simply don't muck with ability scores.
Instead capture the spirit of the spell: you and the target compare an ability score - the lower gains advantage on all attacks/checks/saves and the higher gains disadvantage on all attacks/checks/saves with that ability score.
Allow an additional save each turn that ignores the disadvantage of this spell.

Such a spell could easily fit in at 3rd or 4th level - it's a restricted version of Enhance Ability (that does more things as well), and has less overall power than Contagion (much lower duration, and you have to target their good stat to give them disadvantage on their good stat)

Answer 3

This answer will assume the spell is being used in a game where attributes are either rolled and then assigned or purchased with a point-buy system. If the spell is used in a game where stats are rolled in order the spell is a little weaker on average, but the utility to a character with any given set of stats will be the same (the likely stat distribution changes, but what attributes each character with a given stat set will target and how good that probably is won't)

This spell should be 2nd level when used offensively

This spell, when used offensively, most penalizes a target reliant on Strength. No casting class you are considering (except the Eldritch Knight, I suppose) cares at all about their strength, so they can safely have as low as 3 in that stat. Kobold wizards can even have a Str score of 1!

The monster most possibly reliant on Strength is the Tarrasque. Replacing its strength score with a 1 changes its strength modifier from +10 to -5 and so reduces its damage by as much as 75 hp per round, unless the DM rules that the Tarrasque uses its constitution modifier rather than strength modifier for damage, but that would be mean. This damage reduction approximately halves the Tarrasque's damage output, which is somewhat better than giving the Tarrasque disadvantage on attack rolls like the otherwise superior second level spell Blindness/Deafness does, particularly since the debuff also stacks with disadvantage.

As a percent of damage, rather than an absolute value, some other monsters are worse effected. The Ogre is the worst I can find so far, with its average damage dropping from 13 to a little over 4-- a 68% reduction.

This is the best the spell can ever be offensively, and it's not reliably better than a similar second-level spell, excepting only its ability to stack with other effects unlike disadvantage/grants advantage debuffs. Considering the spell requires Concentration, that's a hard sell.

The spell is completely absurd when used defensively, changing the nature of the game beyond the effects of any other spell besides possibly Wish

The spell also gives you the score you steal, temporarily. That is okay when the stolen score is an enemy's, but ridiculous when it is used in conjunction with an ally. Let's say my Wizard casts Swap on something with more Int than I have. Then my allied Cleric casts Swap on me. Now I have my ally's Intelligence and he has the monster's. I end my spell and give the monster my ally's Intelligence, then my ally ends his spell and gives me the monster's while I give him mine. Now the monster has my Cleric's dump stat Intelligence permanently, and I forevermore have whatever absurd Int I managed to find on a monster.

Alternatively, let's say I have a Potion of Hill Giant Strength. I quaff said potion, then cast swap on an allied fighter or some such. Now both our scores are 21: mine because I'm still affected by the potion and his because my score was 21 when we swapped. Eventually, the potion wears off. My score drops to whatever my number normally is, but his doesn't-- he was never affected by the potion. Then the spell ends and I go off with a permanent increase to my strength score. If I instead end the Swap spell first, the fighter is the one that gets the permanent increase. This only gets worse if rather than a potion I am possessed of a belt or other permanent increase!

While 5e has a lot fewer ways to increase ability scores temporarily than 3.5 did, and also implements hard-caps on ability scores if one plays with twitter erratas, this sort of Manipulate-Form-esque permanent ability score raising combo is still ridiculously powerful. Being able to alter temporary increases into permanent upgrades is a problem unless your players are already going around with 30s in the stats they care about.

This spell also is ridiculously useful to necromancers

Necromancers can only control permanently undead with a low-enough intelligence score. By having an allied Mummy Lord or similar swap intelligence with me after swapping intelligence myself with a high-int undead monster, I can save myself a lot of the trouble that normally comes with enslaving powerful undead beings.