Stalleo has lost his fort to bandits. Slip in through the escape tunnel, fight up to the courtyard and pull the lever that opens the main gate for him.
Speak to Stalleo at his camp across the river from Treva's Watch.
Collectibles from this quest
Detect Life – Alteration: Stocked by Tolfdir at the College of Winterhold and Wylandriah in Riften once your Alteration reaches 40. Stalleo also hands one over at the end of Infiltration.
