There was one chance to save Gundren from being sacrificed: to complete a task for the Kuo-toa tribe which held them captive. Specifically, they were to kill an evil one-eyed creature and retrieve a magical staff. Should those objectives be achieved the Kuo-toa would create a teleportation circle for the party to exit the Underdark with Gundren.
Through various trials and winding tunnels the party traveled, until they came upon a large golden portal suspended in mid air. On the other side, in an enclosed arena, a Death Tyrant waited for them, guarding a magical staff. It was there that our heroes made their stand. The Death Tyrant finally fell - but not alone. Kaelthiros and Murry both had been reduced to little more than a pile of dust by great beams shot from the Tyrant’s eye stalks.
The ashes of Murry had been sucked back into the now-sealed bottle to which he had been attached - immovable and unbreakable. However, Dain was able to pocket some of the ranger’s ashes. Perhaps he could still be resurrected . . . time would tell.
The remnant traveled back to the Kuo-toa tribal village with proof of their deed, and the magical staff in hand. True to their word, The Kuo-toa allowed the party to exit the Underdark with Gundren.