The Lake of Lusor was a body of water located on Qo'noS, the Klingon homeworld, near the Kri'stak volcano.
According to Klingon religion, after plunging a lock of his hair into lava from the volcano, Kahless the Unforgettable forged the Sword of Kahless, the first bat'leth, by cooling the sword in the Lake of Lusor. (TNG: "Rightful Heir")