The right tunnel, which slopes upward, does not go that deep into the mountain. It goes about a hundred feet before turning sharply to the left, then spirals upward until it reaches a large cave about a hundred feed across. In the cave are giant crystals that reflect the light from a small opening in the mountain. It sparkles brightly, lighting the tunnel beyond for a good twenty feet, even though the tunnel curves. A dark druid lives here with his bats and giant spiders. He was corrupted by a demon when he tried to save the creatures of the surrounding forest. Though he killed the demon he was tainted by its dark magic and now seeks to destroy everything besides his forest creatures. The bones of past adventurers and explorers litter the grounds, so there may be some good treasure. Also, the crystals would be good components for magical items. The players just have to defeat the dark druid.
I would suggest what ever level is comparable to your characters, but one higher. Treasure is up to the GM. Since it's so close to the surface, the adventurers may just find adventure gear and very little "treasure".