Regret

Regret is Disc 1, Track 15 on the Xenoblade Original Soundtrack. It plays during these cutscenes:
 * Sharla tellking Shulk and Reyn about the Mechon assault on Colony 6 in the Refugee Camp
 * Sharla's nightmare about Gadolt and Juju just before heading out for Colony 6
 * Shulk's discussion with Otharon, proposing to observe the Mechon after his Vision in the Ether Mine
 * Melia reporting to Sorean after she returned from Makna Forest
 * Face Nemesis meeting the party on Valak Mountain
 * Just before the party enters Sword Valley
 * Miqol telling the party about Egil inside on the Fallen Arm
 * Meeting Vanea after the battle with in Mechonis Field
 * The party meeting Egil in person at the Meyneth Shrine in Agniratha
 * After regrouping to Junks again before heading out for the Mechonis Core
 * Egil telling his backstory with Arglas and Zanza in the Mechonis Core
 * Reyn arguing with Alvis inside Junks about Zanza
 * Tyrea's return after defeating the Mysterious Telethia in Makna Forest during the Melancholy Tyrea