Sergeant Dalen was one of the defenders of Bastion's Keep. During the siege on the keep by Azmodan, he encountered the Nephalem, who had succeeded in lighting the signal fires. A vision of Azmodan appeared to mock their efforts, but the Nephalem assured Dalen it was just a vision, designed to undermine their morale. With that done, Dalen directed the Nephalem to the Stonefort, where the keep's catapults could be raised.[1]
In-game
Dalen is an NPC in Act III of Diablo III. He is found at the end of The Siege of Bastion's Keep quest, and starts the Turning the Tide quest.