The Mark of Shadow grants divinatory powers of clairvoyance and scrying, as well as illusory powers of deception.
Houses: One of the most surprising and devastating effects of the Last War was the schism of the family that carries the Mark of Shadow. House Phiarlan, an elf family of ancient lineage, carried the Mark of Shadow for millennia before the Last War and was known as an information clearinghouse. During the war, however, the mission of the house turned its members against each other, eventually resulting in the formation of a new house—House Thuranni.
The two houses continue their ancient work of espionage, surveillance, and similar intelligence-gathering activities, while outwardly pursuing mercantile activities based on entertainment and artisan endeavors. House Phiarlan’s Entertainers and Artisans Guild is based in Breland, and its spies work for (or sell information to) Thrane, Aundair, and Breland. House Thuranni, based in the Lhazaar Principalities (but with an enclave in Breland), controls the Shadow Network. Its members perform their spying activities on behalf of the Lhazaar princelings, Karrnath, Droaam, Q’barra, and even the Order of the Emerald Claw.
Unmarked members of both House Phiarlan and House Thuranni work as entertainers and artisans who gather information and serve as spies and rogues.
Source: Eberron Campaign Setting