Following the lava tube past the hellhounds, it gets hot. The party enters a large room with glowing rocks. The floor is cooling (but by no means) cool, lava. Inside "cooling off" there are three Fire Nymphs.
The nymphs look tired, but they do not attack. Nor do they attempt to charm. The floor is too hot to wal across and the nymphs move further back.
They will ask the party what they want, and try to figure out if they have a ruby or diamond of good size (50 gp). They will say they missed their opportunity to get back to the Plane of Fire and are stuck here. If the party has a ruby or diamond, they can use a ritual to get back home. The gem will be destroyed though in the process.
If the party coporates with them and lets them have a gem. Then they should receive the same XP as if they had defeated them. If there is a party member with Charisma of 16 or higher they will offer a "Kiss of the Fire Nymphs" to the party. This will grant anyone who receives it a +1 against all saves vs fire and fire magic for 24 hours.
If the party chooses to attack the nymphs, instead they will use all their burning hands attacks and try to charm anyone onto the lava floor. Grant the party only 1/2 the normal XP for defeating the fire nymphs in combat.