Lieutenant Lavail was one of the defenders of Bastion's Keep. During the siege on the keep by Azmodan's Army of Sin, he encountered the Nephalem, who had been assigned to raise the keep's catapults—something that had previously been impossible due to the number of demons present on the keep's walls who had succeeded in lighting the signal fires. A vision of Azmodan appeared to mock their efforts, but the Nephalem assured Sergeant Dalen it was just a vision, designed to undermine their morale. With that done, Sergeant Dalen directed the Nephalem to the Stonefort, where the keep's catapults could be raised. Alongside the Nephalem, Lavail fought off the demons long enough for recruits to raise a catapult.
Dalen is an NPC in Act III of Diablo III. He starts the Tremors in the Stone quest and when asked the whereabouts of Adria, Leah and Tyrael during the quest Heart of Sin, he informs The Nephalem of their location.