Imp (Challenge room)

The Imp is encountered during Baldur's Gate II: Enhanced Edition or Baldur's Gate II: Throne of Bhaal.
He'll be encountered in the Watcher's Keep – Challenge Room (Imp) accessed from the Watcher's Keep Final Seal dungeon level, which is part of the Watcher's Keep adventure.
This creature is essential to the Watcher's Keep plot and necessary to advance the Unlocking the Final Seal questline.
The creature can't be harmed under normal gameplay conditions and will never turn hostile. If somehow the creature is destroyed (through cheats/exploits, console commands etc.), the player's storyline will be shuttered, and the plot broken.
==Gameplay==
The imp will initiate dialogue with the newly arrived party automatically, even if the party is invisible. 
No orders can be issued to the party characters, only dialogue reply choices are available until the challenge is resolved successfully.
The actual challenge/riddles/games that the party is presented with are covered on the gameplay section of the challenge room article.
When the challenges are solved satisfactorily, the imp disappears from the area map and is gone from the game forever.