The Lair is located at 1183S 1583W, roughly halfway between Dernholm and Black Root along the coast. It is marked on your map once you've accepted the melee mastery quest from Sir Garrick Stout of Dermholm.
As indicated, the area is guarded by several Gyr Dolours who are guarding Lady Druella (Point 2). Your goal is to rescue her starting from Point 1, extracting her from the Gyr Dolours whilst keeping the Lady alive. If successful, she will become a temporary follower until you deliver her to Stout for your training.