Mnevis is a Tok'ra operative spy in Ra's degrees. Mnevis Ras was most faithful followers was a young Goa'uld servant. Originally a servant of Hathor who helped manage his Jaffa. Mnevis became known for his "sixth sense" that allowed him to stage attacks to thwart such surprises and turn potential attackers. After years of dedicated service, Hathor gave her loyal servant of Ra to win his favor and soon Mnevis commanded Ra's own strengths.
At Mnevis ability to predict movements enemies became even stronger. In fact Mnevis got insight into everything from combat to policy precognition that the other System Lords could not understand. Mnevis soon became known as "Mnevis Oracle" and could no longer spend time on the battlefield because of the responsibility placed on him. The truth is a little less mysterious: Mnevis was a Tok'ra spy who used this public information networks to advanced word troop movements from other spies in the service of other System Lords.
When he got Ra he seized the opportunity to advance Tok'ra cause and began to feed information from the Tok'ra network to its new master in order to win his trust. After Mnevis took victory after victory in Ra's strengths, he could serve as a full-time operational information, channel news Ra's chips in exchange for giving daily "visions" and "prophecies" to System Lord. Since none of the Tok'ra could anticipate re-opening of the Earth Stargate was Mnevis not warn Ra his appearance and was forced to flee back to the central command of the Tok'ra to work as coordinator of the spy network. Today, he is in the service of any Goa'uld working secretly to undermine them, but he can. (RPG: "Stargate SG-1: Living Gods: Stargate System Lords")