|This has no clear connection to the Forgotten Realms.
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Rule-of-Three was just about the most knowledgeable source on the Abyss available. He dwelt on Azzagrat in the city of Zelatar or in Sigil. He was allied with Graz'zt, and knew much about the Abyssal Realms. When in Azzagrat, he stayed in the back room of an inn called the Golden Opportunity. People searching for information from him could easily access it, but for a three-pronged price, such as precious metals of three different types or three magic items.
Rule-of-Three was the mastermind behind the plan for which he was named, a uniting of tanar'ri, yugoloth, and baatezu to take over the upper planes.