There's a time limit in this quest.
If you invite Montaron and Xzar into your party, they will insist on going to Nashkel as soon as possible. They agree to a quick detour to the Friendly Arm Inn area, but will become increasingly agitated as time passes if you do not go there. They will eventually leave the party if you stray too far from their goal for too long. It is not necessary to complete any quests in Nashkel to get them to stay. Just visiting the area once is enough.

After about 15 days game time without reaching Nashkel:

Nashkel is where I must go. If ye will not go there yerself, then here is where we part company! Sleep lightly, if ye dare to sleep at all!

— Montaron, as he and Xzar leave the party

In your JournalEdit

  • Triggered By: Inviting Montaron and Xzar into your party.
  • Journal Section: Quests
  • Quest Title: Xzar and Montaron
  • Entry Title: Xzar and Montaron
  • Quest Begins: 
    Xzar and Montaron wish me to travel to Nashkel.
  • Quest Completed: Special. When you arrive in Nashkel, this quest is simply removed from the Quests section of your Journal, with no entry in the Done Quests section.