What is the appropriate spell level for this homebrew spell (Devolve Person), intended as a mental counterpart to Hold Person?

Question

Overview

I recently became interested in exploring physical vs. mental spells in D&D 5e and I noticed that there is no true counterpart to Hold Person. Specifically, to my knowledge, there's no spell which result in automatic failures on mental saves (in contrast to Hold Person, which, via the paralyzed condition results in automatic failures on STR/DEX saves).

So I decided to make it. However, I'm not quite sure what should be its spell level.

On its own, its quite weak, however in conjunction with other spells I feel that it could become quite strong. As such I'm currently leaning towards 2nd or 3rd level, in line with Hold Person.

The Spell

Devolve Person

Casting Time: 1 action
Range: 60 feet
Target: A humanoid that you can see within range
Components: V S M (a piece of flesh)
Duration: Concentration, up to 1 minute
Classes: Bard, Cleric, Druid, Sorcerer, Warlock, Wizard

Choose a humanoid that you can see within range. The target must succeed on a Charisma saving throw or lose control of its intellect, reducing it to a beast-like state. At the end of each of its turns, the target can make another Charisma saving throw. On a success, the spell ends on the target.

While in this state, an affected creature's actions are controlled only by their primal instincts. They cannot distinguish friend from foe and any actions that they take must be in interest of their own self preservation. Additionally, they have disadvantage on all Intelligence checks and checks involving social interaction, and they lose the ability to perform complex intellectual tasks like casting spells and speaking.

Moreover, they rely unquestioningly on their senses. As such, they automatically fail Wisdom and Intelligence saving throws.

Balance Callouts

• I've chosen Charisma as the saving throw for thematic purposes, as I imagine targets needing to stubbornly hold on to their fleeing intellect. This naturally makes it a bit stronger as Charisma saves are more rare.
• I deliberately included "self preservation" language to make it clear that the target doesn't go berserk and attack everything. i.e. the target still acts in their own interests, just without the ability to use reason.
• As with Hold Person, this only works on humanoids, it has a short duration, it requires concentration, the save is repeatable, and nothing happens if the target succeeds in their save.
• For the time being, there's no scaling with spell levels.

Answer 1

Start with spell level 2

This spell is stronger against a single strong opponent, and weaker against multiple opponents that have less hit points.

The functional effect is comparable. This will make the target vulnerable to be hit by additional spells. This is stronger for removing a single, strong opponent with a two-spell combo. For example, you could automatically remove them with Polymorph. As a tradeoff, it does not make it easier to hit the target, while the paralyzed condition of hold person makes the target vulnerable to damaging attacks.

The charisma saving throw makes this stronger than hold person. Not only is it a more rarely used save than wisdom, the average monster in the monster manual up to challenge rating 10 also has a lower chance to make this save based on their ability scores, of expected 68% fail rate vs 61% fail rate. This is a meaningful difference (and the calculation did not include save proficiencies which are more common for wisdom than charisma so it might be even higher). The difference shrinks for higher CR level monsters.

On the other hand, you left off the at higher levels option of hold person which is obviously making this relatively weaker, as you cannot remove multiple opponents with one action. I think this evens out the higher power of using charisma saves in some way.

All the other parameters seem to be the same as for hold person.

In comparison, banishment at level four is also able to remove an opponent from combat with a charisma save, but it has no resave chances and it can be cast at higher levels to affect multiple targets, and not only humanoids.

There are also non-combat applications for this spell, making it easier to force through a wisdom-based effect like charm person, or suggestion.

The main balance concern here would be the auto-fail on Wisdom saves (effects that call for an Intelligence save are extremely rare). The creature still gets a Charisma save, so typically the primary effect is to decrease the probability to make that save, for the cost of 2 spell slots and 2 actions, that seems not too powerful. You could just as well try and directly cast the Wisdom save based spell instead. Think of it this way: if you had a feature that allowed you to change the Wisdom save on your next spell to a Charisma save, in exchange for a full action and an extra spell slot, would you use it? The main exploit I can see is to burn lower level spell slots here to make sure a higher level spell like feeblemind will succeed.

I think this should be spell level 2, as hold person is. However, as you say yourself, this might be level 3, and it would be a good idea to playtest this.

Answer 2

I unfortunately don't think this spell is ok at any level

As such, they automatically fail Wisdom and Intelligence saving throws.

That is unfortunately just broken in my view.

One failed save (and Charisma is a good save to target) and they can get hit by all sorts of nasty effects. They are effectively out of the combat after 2 spells, one of which would likely be the same AoE that a party member was already going to cast, so this just set up a really nasty knockout punch.

Hold person is balanced because the allied melee attacker has to get close up, which puts them within reach of allies, and means they are in danger if the target succeeds on a subsequent save, but spells are almost always ranged so they don't have that downside.

You also have to consider that a single critical hit tends not to completely win a fight, but a well placed save or suck spell does.

Suggestions

Change it to disadvantage, and you have a powerful spell, probably on par with hold person, and good for testing in a level 2 slot.

Warnings

Be careful with upcasting if you decide to add that. For hold person melee attacks aren't often AoE, but spells that target saves are, so this becomes exponentially more powerful, unlike hold person.