Stone Speaks to Stone. Greenwing & Dart, #1.5

Jack Greenwing is good at scouting. He's renowned for being able to hold on in battle long past bravery. He was once given a trophy for his courage by the hand of the Emperor himself. At Loe, where the Stone Speakers can call down avalanches and landslides, he was sent out on a scouting mission as a siege closed in on his company. He returns to see it being lifted because of treachery, and is there to witness the five remaining members of the command staff be led off in chains into the mountains, far from the border of the Empire. He once held a border until the rest of his army joined him: nothing will stop him from attempting a rescue, though snow is falling in the mountains and he must fight those who can walk through stone. "Stone Speaks to Stone" is a standalone short story related to the Greenwing & Dart series.
If you've read those, it is the true tale of what happened to Jemis' father at Loe; if you haven't, Greenwing & Dart tells the story of what happens to Jack's son Jemis ten years or so after this story takes place.