You set up a magical boundary around your lair. The boundary can’t exceed the dimensions of a 100-foot cube, but within that maximum, you can shape it as you like—to follow the walls of a building or cave, for example. While the spell lasts, you instantly become aware of any Tiny or larger creature that enters the enclosed area. You know the creature’s type but nothing else about it. You are also aware when creatures leave the area. This awareness is enough to wake you from sleep, and you receive the knowledge as long as you’re on the same plane of existence as your lair.