You have to reduce their Intelligence below the threshold
You will need to reduce their Intelligence in order for this to work.
Wishcould potentially be used to do this, but may come with its own consequences for a request like that.
Here are a few more options.
Both of the options below require giving an NPC an immensely powerful item or arranging for a very strong relationship between the NPC and a demon lord.
Giving an artifact with major detrimental properties to the creature
The DMG has an option for DMs to give artifacts (and other magical items) beneficial and detrimental properties:
Each artifact has its own magical properties, as other magic items do, and the properties are often exceptionally powerful. An artifact might have other properties that are either beneficial or detrimental. You can choose such properties from the tables in this section or determine them randomly. You can also invent new beneficial and detrimental properties. These properties typically change each time an artifact appears in the world.
An artifact can have as many as four minor beneficial properties and two major beneficial properties. It can have as many as four minor detrimental properties and two major detrimental properties.
If you were to gain access to an artifact, and have it exhibit the following major detrimental property, then it could work:
When you become attuned to the artifact, one of your ability scores is reduced by 2 at random. A greater restoration spell restores the ability to normal.
However, you would need get an artifact and have the creature attune to it. And have either the DM provide this property or have it randomized.
Giving a deck of many things to a creature and hoping they pull the right card
Thomas Markov's answer suggests this as another possibility. The Idiot card in the deck of many things says:
Idiot. Permanently reduce your Intelligence by 1d4 + 1 (to a minimum score of 1). You can draw one additional card beyond your declared draws.
Mordenkainen's Tome of Foes relates that NPCs/Monsters may given a boon by the DM in relation to a Demon lord. The Jubilex is able to offer:
Up to a +8 bonus to Constitution, with an equal penalty to Intelligence, Wisdom, and Charisma