I personally think this is a good spell for wold shaping.  The first use that comes to mind is the powerful black dragon Galbatorix’s bound as his mount (in Aragon).

Breaking the mind of a mighty beast which would usually be too proud to serve the dark lord, is both tone setting, and an excellent mechanism for putting unlikely monsters on the big bad’s side.

I know this has been around since 1e, but I hope you all noticed the material components: marbles.... Gotta love Gygax humor. 

Monks at 14th level (which you'd realistically probably be at if you're facing something with eighth level spells) also get proficiency in INT saves.  Also, a Paladin's Aura of protection can make the save more possible by adding their CHA modifier if the affected player is within 10 feet, without having to know when the save is going to occur.

This is an automatic hit just like magic missile AND it basically turns ANYONE into a baby...

I AM GETTING THIS SPELL

I would say that rolling a natural 20 on the saving throw would be grounds for ending the effects of the spell. But otherwise, I agree with you that the saving throw after the first failure is effectively pointless because there is no feasible way of making the INT save after being reduced to 1 INT. Unless if they just really want to try and hope for that natural 20.

So my players ****ed with a bandit queen and stole a holy relic that she wanted while in the middle of a trade amongst a nation of Drow, I had one of her mages cast this on one of the party members, and let's just say he was pretty pissed, for a while. 

Quote from BirdMonk >>

This is an automatic hit just like magic missile AND it basically turns ANYONE into a baby...

 

I AM GETTING THIS SPELL

 
It's not an automatic hit like Magic Missile.  It has a saving throw.  

Quote from Toxxus >>

One aspect I feel is often overlooked on this spell is that it is effectively permanent.

The save once every 30 days - in most cases - is pointless.

If you have an INT score of 1 you're making the save with a -5 on the roll.  The best you can get is a 15 (20 - 5).

The caster who put the Feeblemind on you in the first place had to be level 15 or higher (+5 proficiency bonus or higher) and even with a primary casting stat of 16 (mostly certainly 20 by that level) you're looking at a spell save DC of 16+.

AKA - Nobody under the effects of Feeblemind can ever, ever pass the save to escape from the effects of Feeblemind.  It's is - in almost all cases - permanent.

 
It can be ended by Greater Restoration, Heal, or Wish.
It can be ended if your party has a Level 9 Cleric, Bard, Druid, Celestial Warlock, or Divine Soul Sorcerer.  Or if they can find a NPC with the Greater Restoration or Heal spell.


Please, just read the spell description!

How does this spell interact with the Headband of Intellect?

Quote from gabrielrockman>>

Quote from BirdMonk>>

This is an automatic hit just like magic missile AND it basically turns ANYONE into a baby...

I AM GETTING THIS SPELL

It's not an automatic hit like Magic Missile.  It has a saving throw.  

I think what they meant is that the 4d6 psychic damage is unaffected by the saving throw. It always does 4d6 psychic damage, and if they fail the save, the rest of the spell happens. “The target takes 4d6 psychic damage and must make an Intelligence saving throw.“ No half damage on a success or anything like that, just 4d6 psychic damage.

Headband overrides it, I think? It magically enhances your faculties, even if you have been turned into a baby. You still have to deal with 1 wis and 1 cha, though.

I cannot figure out how to get the Character Builder in DNDBeyond to allow for an INT and CHA of 1, 3 seems to be the minimum. Anyone have any ideas?

Build it as a cursed item that applies the appropriate penalties. You don't really want to lose track of your original stat, with a 1 INT you probably wont remember what it was. :D

@ChrisHarp, thanks, thats a great idea.

When you cast this on a vampire as a level 15 Necromancer and use Command Undead to have it infinitely under your control.

Quote from BadhandsBob >>

By consequence let's all consider, if you "can" dispel magic and remove Feeblemind, then you probably "could" also dispel magic and instantly kill a duplicate created by the "Clone" spell. Another 8th level spell, with duration of instantaneous. That does not seem correct.

One could reason that the 30 day period is representative of recovering from a debilitating mental injury. Your body is healthy but your mental state is soup in a blender, you got some horrible PTSD or something. You don't need to just instantly get better after a save on 30 days, the GM could naturally ease your character back into it, seemingly making a recovery. The problem wasn't that you were suppressed by magic, the problem was you got your brain tasered and temporary lobotomized by magic. Someone attacked your ability scores and damaged them, dispel magic doesn't repair damage.

 BadHandsBob back at it again, being the greatest.

(I saw you posting on the storm of vengeance spell as well).

I think that is a great feature. Maybe increase the hit points required to remove it though because it is such a high level but that's my suggestion.

please no more arguing. dms can control how this spell works in any way so dont criticise them for misunderstanding

Does it say it can be dispelled?

If the target fails the save at the end of 30 days, does the spell keep going for another 30 days until they can save again, or does it end?

Quote from RedHood5229 >>

If the target fails the save at the end of 30 days, does the spell keep going for another 30 days until they can save again, or does it end?

 Keeps going. Basically forever until they make their save. It says "At the end of every 30 days, the creature can repeat its saving throw against this spell. If it succeeds on its saving throw, the spell ends."

If it fails the save then it would be odd if the spell ended in the same way it would have if they made the save.