Can you use True Polymorph and an Intellect Devourer to potentially learn Deep Speech and a secret?

Question

I have been thinking of making a homebrewed campaign where, as part of a bigger plot, the party needs to learn Deep Speech and a secret that a particular Intellect Devourer learnt from consuming the brain of a king from long ago.

Can True Polymorph be used twice -once on the Intellect Devourer and once on the party member- to potentially "extract" Deep Speech and the secret from the devourer? (Players Handbook, p.283; Monsters Manual, p. 191)

The plan would be to cast True Polymorph on the Intellect Devourer to turn it into a humanoid of some sort with low Intelligence (a Gnoll or Troglodyte, possibly). Then cast True Polymorph to turn one of the party members into an Intellect Devourer. The polymorphed party member then uses Body Thief to replace the polymorphed devourer's brain. Finally the polymorphed party member reverts back to their normal form and would know Deep Speech and the secret.

Does this work?

Answer 1

Yes this works for the secret and deep speech temporarily

Once you have occupied the humanoid form of the intellect devourer, you gain it's knowledge and deep speech:

It knows everything the creature knew, including spells and languages.

However, you would lose access to deep speech if you exit the body and dispel true polymorph as you would no longer have the Body Thief feature. You could likely memorize a portion of the language but memorizing the entire language would essentially be training for that language.

That said, the secret would be easy to either memorize or just communicate to the party once you have stolen the intellect devourer's polymorphed body.

But why go to all this trouble

All that said, it would be considerably easier to aquire the secret and deep speech a different way.

The 2nd level detect thoughts thoughts spell could extract the secret without a save in some cases (like if you knew deep speech and were interrogating it):

You initially learn the surface thoughts of the creature - what is most on its mind in that moment... Questions verbally directed at the target creature naturally shape the course of its thoughts, so this spell is particularly effective as part of an interrogation.

...or in the worst case, on a failed wisdom save:

If you probe deeper, the target must make a Wisdom saving throw. If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates).

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You could get deep speech just by training:

You can spend time between adventures learning a new language or training with a set of tools. Your DM might allow additional training options.

First, you must find an instructor willing to teach you. The DM determines how long it takes, and whether one or more ability checks are required.

The training lasts for 250 days and costs 1 gp per day. After you spend the requisite amount of time and money, you learn the new language or gain proficiency with the new tool.

...or via a combination of the 1st level comprehend languages and/or the 3rd level tongues spell if you need it in a pinch.

Answer 2

Frame challenge: You are writing a script, not a scenario.

This is a script that the players are expected to follow. Not a game they are expected to tackle by themselves. Don't prep plots but let the players come to their own resolution.

• They might have a different spell in mind that will pluck the information needed.
• Maybe a player even knows Deep Speech already. As part of their background or somewhere else.
• Maybe they decide to befriend the Intellect Devourer.
• Or even, and this is also highly probable, they ignore this plot thread.

Making your whole campaign hinge on, frankly, rather convoluted solution that is players have to take is not a good way to plan.

When information is needed, then consider the three clue rule. Perhaps the intellect devourer is only one of the possible ways to learn the secret. Here are other possibilities:

• The king have a journal that contains it. A black market dealer offers it to the players for a little favour.
• The secret is still kept by a descendant of the king's close advisor. Yet, they have recently have gone missing.
• A long-lived being was a witness. Maybe a demon or a celestial. And it can be recruited to share it. For a price.
• The king's ghost can be contacted. There is a necromancer who knows how to do that. Are the players willing to enter an agreement with one?
• Whatever the secret is and how the king came to know it is also very likely known to others. Maybe whoever relayed it to the king can be contacted, maybe if the secret relates to a location somebody else witnessed it, maybe if the secret is about some person they can be found or interrogated.
• There are spells such as Commune or Contact Other Plane or Legend Lore which can be used to track down the secret. Even if the players do not have access to those other characters might. Finding one and convincing them to help can be a mini-adventure (or not "mini-") by itself. Moreover, that adds the exciting plot hook that somebody else would also know the secret.

There are many alternative routes to get to the information. Let the players decide and aid them in their decision.