“The Sword of Kahless” is episode nine of season four of Star Trek: Deep Space Nine.
Dax’s friend the Klingon master Kor, last seen in “Blood Oath”, has come to Deep Space 9 again. He has found details of where the lost sword of Kahless is. Kahless was the first Klingon emperor who unified the planet, but his sword, the first bat’leth, was stolen.
Kor wants to recover the sword to help reunite the Klingon Empire under the clone Emperor made in the TNG episode “Rightful Heir”, rather than simply under the High Council. Worf wants to go with them, and does. The stolen sword has been possibly discovered in some ruins on a planet in the Gamma Quadrant which were explored by Vulcans.
They aren’t the only people looking, though. Kor has been less than discrete, and Toral of the House of Duras, whose life Worf spared in the TNG episode “Redemption: Part II” is also looking for it.
Although they find the sword, doing so causes problems. Worf and Kor turn on each other, not considering either worthy to hold it. Nor do they really consider anyone else to be worthy.