In The Book
In Immortal Guardians, notably Takoda, Kovo, Olvan, Conor, and Meilin travel to these mountains to find a sketch that Kovo drew in charcoal (a fanged jaw-shaped doorway). According to a book located in a secret room of the Greenhaven library, the sketch is located in the Petral Mountains.
The Greencloaks set out and find an abandoned city that had been abandoned for a long time. Evidently, the people who inhabited the city didn't want anyone to find the jaw-shaped doorway, as they surrounded the door with a trapped moat. When the Greencloaks enter the room behind the doorway, the tunnels start to collapse because Kovo had broken the door that had stabilized the rocks.
The mountain range is not mapped or spoken of in any other books.