The Ceremonial Dagger is a Yautja weapon, used by warriors as a last-resort weapon or a weapon to take trophies from fallen enemies. The Ceremonial Dagger is associated with the Blooding Rite of Young Bloods. It's primary use is to remove the carapace of a Xenomorph but it could also be used as a close-contact weapon if need be. The blade itself is made of either the bones or the 'resin' of a Xenomorph, making it resistant to their acidic blood.
It is first seen in Alien vs. Predator when the Yautja, Celtic, attempts to finish off the Xenomorph Drone, Grid before the Xenomorph can escape from his net. However, Grid bursts free and Headbites Celtic. It is seen again later when Scar carves up a dead Xenomorph apart to fashion weapons for Lex.