Serpent's Trail is the only passage to the Lost Valley, which contains the Pale Pass ruins. This trail is part of the side quest "Lifting the Vale," and contains Rats, Wolves, or Bears depending on level, and Pale Pass Ogres, also level dependent.
Quests[]
Lifting the Vale[]
When the Hero enters Bruma, a Nord named Tolgan will ask them to meet with Countess Narina Carvain and give them 25 
as an incentive. She needs some help in finding an artifact.
When the Hero speaks with the Countess in Castle Bruma, she tells them that she's an avid collector of Akaviri items and considers her collection the most complete in Cyrodiil.
Read through the Akaviri Diary Translation given by the Countess because it will give some history about the location and about the quest. The messenger's body will be in the cave, and make sure to grab the Akaviri Orders from his skeleton's hand.