You'll encounter this secondary quest intertwined amidst the primary quest during step three of Act I's Chapter 9 (Windstone Fortress). All that remains of Squadron Leader Taarth's squad are three soldiers, who managed to survive the conversion to plagued enemies. You'll likely run across one or two (and possibly all three) of them, before finding Squadron Leader Taarth.

Private Banos is found hiding in the first room along the south wall of the first hallway almost immediately after your party has entered The Eastern Windstone Fortress Gatehouse via the rear entrance. Speak to him and agree to escort him to initiate the quest. If your praty comes in from the other direction, speak to Squadron Leader Taarth and agree to look for his missing men before entering The Eastern Windstone Fortress Gatehouse from the street to initiate the quest.

He won't help your party fight their enemies, but he can't be killed by them either. If your party is taken out on the way, he'll be waiting where they died, for their return. Although there are a great many side areas in the gatehouse, the basic corridors form a "C". Just follow them around and out the back door of the Eastern Windstone Gatehouse. Once there, continue out and walk north down the street and find Squadron Leader Taarth. As they near Squadron Leader Taarth, Private Banos runs ahead to greet and faill in behind him. If your character speaks to Squadron Leader Taarth, he'll just say "Go find those men!", meaning Privates Caiden and Nolan.

The entrance to The Windstone Fortress Barracks is southwest of and across the street from the teleporter. Enter and head west, then continue all the way to the rear in a straight line to encounter Private Nolan hiding in a room. After speaking to him, he will follow your party back out of the barracks and back to Squadron Leader Taarth.

After dropping off Private Nolan, head south toward the fortresses' main gate. Head west up the steps, just before reaching the blocked main gate and enter The Western Windstone Fortress Gatehouse. Turn south once they're inside and proceed all the way to the rear. They can pick up "The_Deeds_of_Xeria" from the table along the west wall just before encountering Private Caiden, who's hiding in the southwest corner. Note that he can't get out to follow them, unless your group first destroys the urns blocking his way. After speaking to him, he'll follow your party back out of the barracks and back to Squadron Leader Taarth.

Return to and speak with Squadron Leader Taarth, who's standing in front of the three privates, after they've returned to the area near the pit where Sartan is being held prisoner, to receive the Dusty Key and complete this quest.