Hail Captain Beltho inside the keep, at ( -2103, -44, 437 ) /waypoint -2103, -44, 437.
Captain Beltho asks you to scout the hidden entrance to the gnoll lair.
Visit the back entrance to Blackburrow, which can be found inside a giant fallen log in a small valley hidden southeast of Archer's Wood. You can reach the valley by crossing two islands joined by log bridges starting with Wooded Isle around ( -765, -39, 988 ) /waypoint -765, -39, 988. The quest should update when you walk close to ( -1214, -3, 912 ) /waypoint -1214, -3, 912.