Finding Your Way

The steppes falling down from the volcano's side are deceptive. They appear to be open and broad, easily accessible and crossed, but they are pockmarked with barriers and obstacles of many kinds.
Finding the orientation of the map you found turns out to be easier than expected, since it flows from the trail you are already following. The map indicates a path running south away from the main trail. Finding it requires a successful Survival roll (DC15), with a 1 in 6 chance of a random encounter for each failed attempt. Likely encounters in this area are a swarm of poisonous snakes, wights, a band of 4-6 troglodytes, a salamander, or 4-6 lizardfolk. A number of other creatures would fit well instead of these if desired, though swamp creatures or fire-based creatures are best. As the party gets nearer the dragon's lair, they will begin to encounter more and more fire-themed monsters (this is part of the dragon's lair effects).
Eventually, the party will find the hidden path. Once they are on it, they drop into a narrow ravine that slopes down into the hardened lava flows, winding back and forth as it snakes forward until it appears to actually dive beneath the lava in a dark tunnel. This tunnel stretches on for 400 yards or more before opening onto a flat shelf surrounded by steep walls, and dominated by a festering pool of bubbling mud and slime. The pool actually runs off the edge of a drop to the right, with a greater morass beneath it. There are rotting remnants of small trees and bushes against the high walls, most with signs of burning in places. There is a particularly dense mass of vegetation to your immediate left, about 20 feet away.
The pool is home to a shambling mound, which is the main part of the mass to your left. It can be perceived as malevolent on a DC18, otherwise it will wait to attack until someone is within 10 feet of it.