The duergar provided him with special glasses that allowed him to see in the pitch black caverns. They had strange lenses allowing him to see enough to navigate the tunnels. Their own eyes were adapted to life underground. They were large, to take advantage of any minimal light, and were able to see heat emanating in the environment. It was unnerving the first time he saw them. It was easy to get lost in those dark orbs surrounded by a sliver of white. The duergar often took advantage of this in trade negotiations, staring down the person across the table.