HoT/Zones/alkaborsnightmare/Defend What?/about
Deputy Asler is on the first island where you zone in (Misty Thicket), atop the wall.
Deputy Asler is the only remaining Rivervale Deputy guarding the wall, and as one of the first adventurers he's seen for days, he has ordered you to protect the wall from intruders. Intruders to which direction is unknown, but there seems to be a lot of enemies around nonetheless.