The dragon isn't hard to find; he's resting in the largest room, on the once-ground floor of the large building, surrounded by treasures and trophies, as any good dragon's lair should be. He'll wake the instant Draiss comes within range of his blindsense, furious at an intruder in his precious lair; when the lizardmen intrude, and they know not to, they're a free snack for him, along with any other creature in the lake.