Reaper is one of the drugs available in Bay City. Takeshi Kovacs bought Reaper, along with a large number of other drugs, from a street dealer. Reaper was developed for studying near death experiences and even a small dose can slow bodily functions like breathing, muscle movement and heart rate. In small doses it renders people unconscious while higher doses can kill.
Kovacs and Vernon Elliot buy Tebbit Knives (which have Reaper cartridges in the blade) from an arms dealer before they go to the party because Kovacs knows Bancroft hired assassins and has a gladiator pit at the party.