So my ~11th level party is going to be interrogating a prisoner (demon, if it makes any difference) to try to get them to reveal their master's evil plan. They'll be doing this in the prison where the demon is incarcerated in a big city.
Ultimately, I would like for this to be achievable, and give information if the party does well, but for it to provide a challenge.
My main problem in this is that one player is a psionic, with access to the psionic equivalent of dominate monster. I would rather this take more than a single obvious spell cast to solve. Also in the party are an alienist and a cleric (no wizard or artificer).
The master will have taken sensible precautions, including casting spells to make interrogation harder. Assume the master has access to any spell up to 9th level. The PCs are also on a bit of a clock: the longer the interrogation, the less up-to-date the information they receive is, and the further along the demon's plans will be.
Obviously, I can use a cop out answer such as "this is all the prisoner knew", but that doesn't provide a challenge or provide the PCs more information the better they handle the situation.
I'm looking either for advice of how people have handled similar situations, or spells/abilities I could employ to make the situation challenging for the PCs more than just "I cast dominate monster until he fails his save (and fails again for performing an act against his nature), then he tells us everything".