The d'k tahg is the traditional Klingon warrior's knife, a dagger with spring-out side blades that also plays a role in Klingon rituals.
Nearly every Klingon warrior carries a d'k tahg, and the weapon is as much a personal token as a tool. Its central blade is flanked by side blades that can be extended, making it a fearsome weapon in a fight.
The knife also has ritual significance. When Worf was left partially paralyzed in 2368 and considered the Hegh'bat, the ceremony of ritual suicide, he asked Riker to help him, the duty traditionally performed by a family member presenting the knife. Riker pointed out that it was Worf's son Alexander who would have to perform that duty, and Worf chose surgery instead.