The harpy trap is a trap in The Witcher 2: Assassins of Kings. It can be purchased from Haggard, Thorak or Earso in Vergen. You need seven of these to fulfill The Harpy Contract quest.
These are used one at a time. To use one, simply arm it and set it down, then wait for a harpy to take it and fly back to its nest. According to a book related to The Harpy Contract, its functioning resembles a bomb; after being taken to the nests of harpies, it explodes.