The D12 class was a type of Klingon bird-of-prey in service to the Imperial Klingon Defense Force in the 24th century. The D12-class was withdrawn from active service in the 2350s because of the defective plasma coils that had been installed, which meant that a D12's cloaking device would often fail or not work at all. (TNG movie: Star Trek Generations; TNG episode: "Firstborn")