Scylla's Daughter

Fafhrd and the Gray Mouser are sent to safeguard Lankhmar's grain ships on a mission that turns out to be far stranger than piracy: they cross paths with a sea serpent ridden by an explorer from another world, and uncover a hidden civilization of intelligent, scheming rats moving against the city itself. Leiber later expanded this novella into the full novel The Swords of Lankhmar.