The Scythe of Muadru is a scythe, over six feet long and featuring three handles, as well as a long, curved blade. Until the events of "The Doomsday Armageddon Apocalypse", the Scythe of Muadru was necessary for mortals traveling through the Underworld. Should such a traveler release the Scythe, even for a second, he would be trapped in that infinitely massive veil of shadow and mystery for eternity.
The Scythe was apparently in the custody of Roxy Wasserman at least through the events of "The Accidental Occidental Conception". She provided it to the Middleman and Wendy Watson for their journey to the Underworld in pursuit of the Terra Cotta warrior.
By the time of "The Doomsday Armageddon Apocalypse", the Underworld had done away with the Scythe (the desk clerk calls it an "ungainly thing") and replaced it with a simple visitor's pass.
Though its primary function was the safe transport of mortals in the Underworld, the Scythe could apparently be used to cut as well. The Middleman used the long blade to cut Duncan free of the Terra Cotta warrior's prison of mud.