Cave of Wonders

(Yes, believe it or not, by name, this is in the ROT sourcebook).

Across the plains is the unusual range known as Mennara’s Teeth. Though the mountains cover a relatively small area, they pierce high into the sky, sharp and narrow, like the fangs of a great beast. It is said that dragons come to these mountains when they feel their life’s end drawing close, and most people avoid the foothills for fear of encountering such a monster. Still, stories persist of the Cave of Wonders, said to lie somewhere amid the dagger-like slopes. Few who seek the cave ever return, and those who do come back changed—strangely serene, yet distant from their old lives, and unwilling to speak of their experience.

Rumor (and appearance) suggest it was made by early Singhara explorers, though who created it (and why) remains a mystery.