Riddle of the Imps - Baldur's Gate 2 Side Quest
Riddle of the Imps is an easy side quest found in the Underwater City (aka City-of-Caverns) map. In order to advance the story during Chapter 4 while you're in the Underwater City zone you'll have to run through this puzzle to get to the Beholder behind it. Completing this puzzle though is completely optional and will reward the player with Boots of Etherealness and Cloak of Protection +2.
For this puzzle what you have to do is speak with each of the illusions that the imps summon and take an item from them. These items will then have to be placed inside of a chest that corresponds to who the item belongs to. If you place the wrong item in the wrong chest there is no penalty for doing so except the fact that the puzzle won't complete until all the items are in the right chests.
Below is a list of each illusion and which item belongs to them.
Khelban Blackstaff: Quarterstaff
For those of you out there who would prefer a picture to explain this information to you, use my screenshot below. I've taken the image icons of all 5 items and placed them overtop the chests that they belong to. Click on the image if you'd like to make it larger.