Intrigue With Jarlaxle - Baldur's Gate 2 Side Quest
The Intrigue With Jarlaxle side quest begins during The Drow Summoning Ritual quest when the city of Ust Natha is under lock down. When leaving the Temple of Lolth for the first time you'll be stopped on your way to the Female Fighter's Society by a man named Visaj who will offer to sell you a magical rope for 1000G. Buying the rope from Visaj will begin this side quest.
With this rope in your inventory you'll be able to enter Deirex's Tower in the southern portion of Ust Natha. The tower is just south of where we met Visaj and begun this quest, It's also marked on your map so you should have no trouble finding it. Enter into the tower and you'll be immediately stopped by Deirex then Imprisoned by him; this will put you into another dimension where you'll meet Jarlaxle.
You will have a long conversation with Jarlaxle inside of this pocket plane but ultimately what he asks of you is to defeat Deirex and obtain some gems that are in his possession. Refusing this quest is not an option since Jarlaxle knows who you really are and threatens to expose you if you don't comply.
After you're done speaking with Jarlaxle he'll teleport you back outside of Deirex's Tower. This time when you enter Deirex's Tower be prepared for a fight against Deirex himself. I strongly recommend you buff up with whatever you got then save and head back inside to fight Deirex.
Important: This quest has a 1 day time limit that starts once you speak to Jarlaxle. Additionally, once inside Deirex's Tower you'll want to loot the entire tower before picking up the Deirex's Gems that we were sent here for. Once you loot Deirex's Gems you'll be teleported back to the pocket plane immediately and any loot you left behind will be lost permanently.
Deirex will drop Lich's Tooth as well as Jae'llat Wardstone when he dies; the Lich's Tooth item is used to access the top floor of Deirex's Tower and the Jae'llat Wardstone is used to access House Jae'llat. Both of these locations are loaded with loot and are your primary "rewards" for finishing this quest.
Inside Deirex's room you'll also find Armor of the Viper +5 as well as many different scrolls, Glitterdust, Wizard Eye, Deafness, Contagion, Remove Magic, Limited Wish, Melf's Minute Meteors and Spook. Once you've collected everything loot Deirex's Gems to continue with the quest.
Back in Jarlaxle's pocket plane you'll find out that he was lying to you (again) and you have 2 ways of proceeding with the rest of the quest. You can threaten Jarlaxle and fight him and Visaj for a total of 26k EXP or you can accept what he did and move on for a total of 10k EXP. Either way the quest will complete here and you will be teleported back to Ust Natha.
Now, as for our "real" rewards for this quest, as stated before you're going to want to visit the top floor of Deirex's Tower as well as House Jae'llat. I've included a list of the loot you can find in both of these locations below. Keep in mind that once you leave House Jae'llat you will never be able to return. For this reason you should collect every piece of loot from inside you want before leaving.
House Jae'llat (Jae'llat Wardstone): The only magical item you'll find in this house is the Water's Edge +3. Though if you search the nook outside of this building you'll also find Bracers of Binding.
You'll get about 100k EXP in total for killing all of the Drow inside House Jae'llat along with some Gems and Gold.