A Folk Tale of the Oasis Basin
There once was a viper who was set upon by an assassin. He survived, but with a deep cut that caused him to move awkwardly forevermore. The viper sought shelter in an underground nest. Vipers do not trust one another–and they are wise not to–but the vipers there were angered by one of their number being attacked. The viper was escorted to the Lord of the Underworld, who made him an offer: if he sacrificed one of his eyes, he could have a magic black opal in its place. The opal showed him what passed before the eyes of the enemy who sent an assassin after him. It was another viper, one with a nest deep within a palace. The viper with the opal eye watched to divine his enemy’s location. He finally felt sure of his enemy’s general whereabouts, and now makes his painful, awkward way up and down the Summer Road, seeking to find himself appear before the eyes of his nemesis.
Excerpt from The Blackstone Dungeon Master’s Guide (1976):
BLACKSTONE: A black opal that lets the user see through the eyes of their most dangerous enemy (as per the spell Niya’s Nemesis Gazing). Effect lasts 2d6 rounds.