This knife is from the Profiles in History Star Trek auction in 2010. It is a hero metal knife, and the only one I know of. There were 5 rubber stunt knives just like this sold through It's a Wrap and you can find those here.
This is the knife used by members of the House of Duras in the early 25th century.
In "Firstborn", Worf and Alexander Rozhenko, while on the Klingon outpost Maranga IV, are attacked by a group of Klingons loyal to the House of Duras. During the fight, K'mtar (actually Alexander Rozhenko from the future) shows up and helps Worf and his son. The assailants flee and one of them drops this knife.
As the knife bears the insignia of the house of Duras, it becomes clear that Lursa and B'Etor are behind the attack and the crew of the USS Enterprise-D starts earching for the Duras sisters. When they are found and confronted with the knife, they claim they know nothing of the attack. When the sisters take a close look at the knife, they see that the knife not only features the small symbols representing Ja'rod (Duras' father), Lursa and B'Etor but also a small symbol representing Lursa's son. As Lursa had only learned a few days earlier that she was expecting a child and had not told anybody but B'Etor, it was clear that this knife must have come from the future. It turns out that members of the House of Duras had traveled back in time from the early 25th century to the year 2370 in order to kill Alexander, just like K'mtar tried to do.
Thanks to the amazing Jorg Hillebrand for this article and screencaps!
Alec