The right under Klingon custom to challenge and kill the murderer of a family member or mate, claimed by Worf against Duras after the death of K'Ehleyr.
The Right of Vengeance is a Klingon legal custom. It allows a warrior whose kin or mate has been killed to demand single combat with the killer, and no one may lawfully stand in the way once the claim is made.
It is invoked in The Next Generation episode Reunion. K'Ehleyr uncovers the truth that Duras's father, not Worf's, betrayed Khitomer to the Romulans, and Duras murders her for it. Worf takes his bat'leth, beams to Duras's ship and claims the right. Duras's men object that Worf is dishonoured and has no standing, but give way when he states that K'Ehleyr was his mate.
Worf kills Duras, which leaves Gowron the only candidate for chancellor. The Klingons regard the matter as closed. Picard does not, and places a formal reprimand on Worf's Starfleet record.