The origins of the Gray Mouser are shrouded in mystery as the Mouser spent his childhood as an orphan. He was also originally known as Mouse. It is hinted that he may have been born somewhere in the lands south of Lankhmar, perhaps Tovilyis.
He was apprenticed to Glavas Rho, a hedge wizard who died at the hands of the men of Duke Janarrl. After being captured by the Duke, and while being tortured, the Gray Mouser used black magic to channel his pain through the Duke’s daughter, Ivrian, to the Duke to killed him. Ivrian and the Gray Mouser escaped to Lankhmar where he met Fafhrd.
Ivrian, along with Fafhrd’s Vlana, was murdered by the magic of Hristomilo, his familiar, Slivikin, and summoned rats as vengeance for the theft of gems from two guild thieves by Fafhrd and the Gray Mouser. The twain have been comrades ever since.