Find Out What Happen With Montaron - BG2 Side Quest
You'll begin this quest shortly after completing the Bring the Poisoned Man to his friends side quest; on your way past the Sea's Bounty Tavern you'll be stopped by Xzar who begins this quest. Xzar asks you to help find out what happen to Montaron, to do this you'll have to gain entry to the Harper building.
Return to Rylock and speak to him, pressure him about this building being the Harper stronghold and he'll agree to let you inside if you do a favor for him first. What you'll need to do is enter Prebek's house in the Docks District and defeat the enemies inside. You can find Prebek's house in the central portion of the docks district, it's one of the homes marked on the map.
Inside there will be 2 mages and some Goblins, search the room for some extra scrolls along with what the casters drop. When they're all dead you'll want to return to Rylock and talk to him, now we'll be able to enter the Harper building. On the first floor of the building all you need to do is grab the Harper Amulet from the desk in the final office room (pictured below).
The Harper Amulet will allow you to safely enter the 2nd floor without being attacked. So long as one character in your party is wearing the amulet everyone will be allowed on the 2nd floor. On the 2nd floor you'll meet a group of ghosts known as the Spectral Harpists who will answer many different questions about a variety of subjects.
I recommend you loot all you can on this building as you won't be allowed inside after you finish this quest (unless Jaheira is in your party). Rummage through the desk in the southern most room and you'll find a note that implies Montaron was turned into a bird. Once you're ready to complete this quest all you need to do is go into the bird cage (pictured above) and click on the top of the trees twice. This will cause a bird to land in your hand who you assume to be Montaron.
Note: You can kill the Spectral Harpists on the 2nd floor for 27,000 EXP and no one on the 1st floor will care
When you give the bird to Xzar it'll be revealed to you that it is indeed not Montaron and actually someone by the name of Lucette. Xzar will be quickly slain and you'll get some dialogue with Lucette explaining that you got played by the Harpers (and Xzar). Once this event is over the quest will conclude.
Completion of this quest is required for Jaheira's Companion Quest to continue. Sometime after finishing this quest, if Jaheira is in your party, you'll be approached by a Harper who will start the next quest in the chain.