The Verchiel March is triggered after speaking to Sera in Skyhold. It's unknown what prerequisites are required for Sera to share this mission, though it's likely related to her disposition. Speak to her and exhaust the dialogue choices and she'll share her concerns with the Verchiel region, which will trigger the quest.
This unlocks a new war map mission on the Orlais side entitled “A Battalion for Sera and Verchiel.” Run the mission however you wish, and when it concludes, head back to Skyhold and speak to Sera. She'll want to leave to get paid for the mission, and she'll want you to come along. Agree, and you'll automatically travel to a remote part of Crestwood.