Ways to help recover:

Bless/resistance: you have to time it within 1 minute, but resistance is a cantrip so you can just recast.

Paladin aura.

Bardic inspiration. You have to time it within 10m.

Headband of Intellect!!!

This is kind of like what happened to Mr & Mrs Longbottom from the Harry Potter series. They are not insane, they are just fragments of their former selves. They recognize someone they love to some extent, but cannot communicate with them. 

Anyone else suspect the material component to be a reference to losing your marbles?

How about using this on a familiar beast, making it possible to use the awaken spell on it, now it can talk Ö

So a creature who fails the save can't activate magic items, but can they use them? Say someone already lit up their Sun Blade. Would it extinguish, or would they be able to keep swinging with it?

Quote from Beik >>

How about using this on a familiar beast, making it possible to use the awaken spell on it, now it can talk Ö

 

 Heh...heh... new intelligent tarrasque companion.

Quote from The_Treebeard >>

So a creature who fails the save can't activate magic items, but can they use them? Say someone already lit up their Sun Blade. Would it extinguish, or would they be able to keep swinging with it?

 

 

Beyond 20

 

 

 

 

 

The description of that blade itself specifically mentions that it's a bonus action to both light and extinguish it.

So no, being reduced to the mental capacity of a baby doesn't automatically extinguish the blade. You have to actively use a bonus action to extinguish it. Not that it matters since if you have an Intelligence score of 1 you've probably stabbed yourself with it by now.

With magic items whose effects last a finite amount of time ("this effect lasts for X minutes"), you would not be able to reactivate them once that time expires if you are still a baby by that point.

As for using them without activating them... I mean I guess you could, but the most you can do with an item you can't activate when you're a baby is mess around with it, much like a baby. Which is dangerous. There's a reason you never trust babies with kitchen knives.

Everyone’s saying this turns it’s victim into a baby, but last time I checked babies aren’t protecting any friends. Heck, they can’t even protect themselves.

Quote from ChemaIX >>

Ways to help recover:

...

Headband of Intellect!!!

 
Stupid question: Is the part
The creature can't cast spells, activate magic items, understand language, or communicate in any intelligible way.
due to the low intelligence or is it part of the spell itself?

If it was the later, a Headband might introduce a few nice chances for roleplay ;)

1 Awaken any animal or plant.

2 Feeblemind

3 Greater Restoration

Repeat 2 and 3 until things get weird.

Quote from Toxxus >>

You make a good point, however, only Druids, Rogues and Wizards get INT Save proficiency.  This leaves the other 75% of classes SOL.

Bless isn't going to be an option unless the friends know EXACTLY when the re-save will occur since it lasts for 1 minute and that means they have to correctly ascertain which of the 1440 minutes of the day will be the one where the save is made.

The reality is that your friends will have to lead you back to town on a leash and pay for Greater Restoration if they don't have a cleric in the party.

 

 thats offensive to my Artificer friends ):

If you stretch "cannot activate magic items" then that could theoretically mean liches and mummy lords being unable to use their resurrection abilities as they are tied to the phylactery and heart respectively. 

This has caused a few issues in our campaign, with the aguement of does feeblemind carry over from a polymorphed person? So an individual polymorph themselves, but then the polymorphed form has feeblemind cast on them. If polymorph is dispelled/spell stops, is the individual then under the feeblemind spell. I'd say no for polymorph, but yes for true polymorph? Thoughts? 

Combo this with Command Undead from the School of Necromancy to have literally any undead you want as a minion

Quote from thedyingart >>

Does the can’t understand language mean that someone can’t be mentally commanded to do something like in the Animate Dead spell?

 Animate dead would work. I’m pretty sure dominate monster would as well, but anything that creates a literal telepathic link to talk to someone rather than simply commanding them would not.

Quote from 11snellj01 >>

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.

 Hit points to remove it?

would this free a creature from the effects of antipathy/sympathy?, like if i were to use sympathy to trap something into an area, then use this would they be able to leave again?

Another one of those "DM hates you" or "cruel DM" spells.  Especially cruel if the spellcaster doesn't rely on Intelligence, or even chooses to dump their Intelligence score.  Time to teach them a lesson!  

a cursed item with feeblemind is the best way to stop murder hobos.

why would it?