The Warhead module was a section of Defiant-class starships used as a last resort weapon.
The module made up the nose section of the Defiant-class, including the deflector dish, and had a separate miniature impulse engine to propel the warhead, armed with magazine of six photon torpedoes, to the target. (DS9 reference: Technical Manual)
In April 2376 Ezri Dax and Nog fired the USS Defiant’s warhead module to destroy the last of three Jem'Hadar fighters which had launched an attack on Deep Space 9. The Defiant's replacement warhead module was sourced directly from Utopia Planitia. (DS9 novel: Avatar, Book One)