Riverbows are deadly weapons designed specifically to kill adult Cactacae, humanoid beings that have the skin of a cactus and the general physiology of a very large human. These deadly weapons are designed like crossbows but are significantly larger in size. The standard ammunition of Riverbows are Chackras, large spinning metal disks with serrated edges. They are one of the only things that can penetrate the extremely tough and thick skin of the Cactacae.
"Riverbows can cleave a human's head from their bodies like a razorblade through butter."
Perdido Street Station by China Mieville