This one has me assuming a couple things. Maybe I need to just ask for examples of how it works but here is my guess:

Lets say an NPC, Violet, casts Mental Prison on a Paladin, Jericho who fails the save. Jericho's team mates are just going to see him looking around like a mime cowering from an invisible maw chewing on him. How does a DM describe this?

The team mates will no doubt yell something to Jericho so unless its scary as heck, he'll try to leave the prison on his turn and take the 10D10 damage...

As a DM how do I describe this thing?

6th-level illusion
Casting Time: 1 action
Range: 60 feet
Components: S
Duration: Concentration, up to 1 minute
You attempt to bind a creature within an illusory cell that only it perceives. One creature you can see within range must make an Intelligence saving throw. The target succeeds automatically if it is immune to being charmed. On a successful save, the target takes 5d10 psychic damage, and the spell ends. On a failed save, the target takes 5d10 psychic damage, and you make the area immediately around the targetís space appear dangerous to it in some way. You might cause the target to perceive itself as being surrounded by fire, floating razors, or hideous maws filled with dripping teeth. Whatever form the illusion takes, the target canít see or hear anything beyond it and is restrained for the spellís duration. If the target is moved out of the illusion, makes a melee attack through it, or reaches any part of its body through it, the target takes 10d10 psychic damage, and the spell ends.