I see how automatic failure could be alarming at face value, however Hold Person already works this way, as paralyzed targets automatically fail Strength/Dexterity saves. This means that ranged effects like Disintegrate automatically hit the target, so both ranged and melee users can benefit from the spell. I think the main balancing lever for Hold Person is the fact that it's humanoid only, which this spell preserves.

@Andrendire I tend to disagree, because spells that target Str/Dex tend to be damage dealing spells, and a creature with enough hit points can still weather that. Spells that target Wis/Int can just straight-up eliminate (or flip!) a target. Feeblemind, Geas, Dominate Person, Imprisonment, Polymorph (in all its forms), Modify Memory, Suggestion. It is, perhaps, telling that there are 4 different Conditions that cause auto-failure of Dex/Str, but no conditions that cause auto-failure of any other Save type.

@guildsbounty this answer focuses on combat, and, in my experience, a pile of disintegrated ash tends to exert little battlefield control. That said, there are certainly out of combat applications which could factor into this, which I'd encourage you to explore in answer of your own.

@Andrendire I was primarily speaking of combat as well. The vast bulk of the spells I mentioned are usable in combat. Geas and Imprisonment being the only two I listed that lack a '1 Action' cast time. Yes, Disintegrate can kill a target very effectively, but they may survive if they have enough HP. But Feeblemind is an immediate end to a spellcaster, and a lot of other wis/int spells immediately remove the target from the fight. No amount of HP can help them.

Addendum: Hold Person is a Wis save. So those two spells stack beautifully. If they fail Devolve Person, they auto-fail Hold Person.

@guildsbounty what I'm hearing is that there are some strong applications when used with teamwork, which is exactly the sort of thing that the game should promote. Regardless, I'll admit that there's clearly some room for modifying wording to account for long-duration effects, but playing mostly Tier 1/2 campaigns I don't find balance concerns around high levels spells like Imprisonment (9th level) and Feeblemind (8th level) to be particularly concerning.

@Andrendire a pile of ash does have little effect on combat, but if a single disintegrate can kill a target they weren't going to have much effect anyway (it is such a poor spell, only really good to escape from a wall of force). A single control spell on the other hand can turn a really threatening monster into an easy one. Damage is important, but only the last 1hp actually does anything, and is waaaaay over-rated normally.

@guildsbounty: Andrendire makes a good point that it requires teamwork to take advantage of this, except with non-concentration spells like Feeblemind. Starting to cast Hold Person or Polymorph ends concentration on Devolve Person, so the target wouldn't auto-fail. I think you're right, though, that there is a high-level balance issue where you can repeatedly try Devolve Person with 2nd-level spell slots until it lands, then do a guaranteed Feeblemind with your only 8th-level spell slot.

@PeterCordes I am not sure teamwork is required, so many powerful wisdom saves in particular come from aoe spells. Yeah it's best use is pre-feeblemind or something, but pre-hypnotic pattern is also great and that doesn't need much planning. I think this spell, and the comparison to hold person show up the difference between melee and spells, melee can get a buff like hold person and not seem broken, but giving a caster a buff like this breaks them. It's not like casters need anything extra to be powerful, they are already top of the class.

@SeriousBri: You still need a 2nd caster for Hypnotic Pattern, which is all I meant by teamwork. If you started to cast it yourself, you'd end concentration of Devolve, which would happen before they had to make a save against Hypnotic Pattern. Agreed somewhat with your point that the other caster may have been going to cast Hypnotic anyway, but some coordination makes sense like delaying until you've attempted Devolve on a high-priority target.

These comments are approaching the point where they should be moved to chat, so I want to reel things in and close this on some (non) acceptance criteria: I can't accept this answer because it ignores the ranged benefits of Paralysis (there are countless ranged spells with Dex/Str saves) and it doesn't answer the root question: what level should this spell be? Feeblemind (8th level) functionally does basically the same thing with other stronger details, so I have to consider "no level is balanced" as an incorrect answer.

@Andrendire: Feeblemind doesn't have any effect on Wis saves. It also doesn't make you auto-fail Int saves, but your ability modifier is -5. (But still, some very high CR creatures have +7 to +9 proficiency bonuses (e.g. a Kraken or gold greatwyrm CR28), so they could still have a +2 to +4 Int save. Not a big chance of success against high-level PCs with a save DC of 19, so the really important point is that Feeblemind doesn't affect Wis saves at all).

Can you name a single AoE Wisdom or Intelligence saving throw?

@TheDragonOfFlame Hypnotic Pattern, Slow, Fear, upcast Hold Person, upcast Tasha's Mind Whip, twinned Dissonant Whispers, Synaptic Static. How many do you want?

@PeterCordes neither of those creatures you list are applicable targets (humanoids only) but they ARE applicable for Feeblemind. Can you clarify why Charisma vs. Wisdom is so important in your comparison? There's a long list of spells with Charisma saves that make for very abrupt ends to a fight.

@Andrendire is there? I can think of plane shift if you happen to hold the tuning fork to a really dangerous place, and that's about it. Banishment kinda, but that only lasts 1 minute so its not a fight ender (but it is massively impactful in the right fight, not saying otherwise).

@SeriousBri Plane Shift is a good one, and Banishment is permanent if it goes the full duration. Forcecage (cha to escape) + Sickly Radiance kills anything. Plus, as we're comparing with Feeblemind, let's imagine that we have a mindless husk. If they're a spellcaster, they can't cast spells and do really anything besides "protecting their allies": what's to stop you from using Magic Jar at your leisure?

@PeterCordes agreed. I'm not denying that it's powerful, but as a counterpoint, Dexterity saving throws are a "major" save, so we already have ample precedent for this sort of mechanic existing via paralysis/stunned.

@Andrendire: As has already been discussed, failing dex saves is usually just about damage, not further / longer-term status effects. Banishment is only permanent on creatures not native to this plane. (Unless the party is off the prime material, that's rare for humanoids). Plane Shift isn't lethal for creatures that can plane shift themselves out again. Symbol can impose "hopelessness" (no attacks or harmful abilities for 1min). Forcecage + AoE effect is one of very few killer effects that depends on Cha saves (or not being able to teleport, e.g. by blocking spellcasting.)

@Andrendire: And note that Hold Person / Monster itself is a Wis save to resist, not Cha. So you get a "major" save to resist paralysis.