Back when Dax was Curzon Dax, he became friends with some Klingons while serving his diplomatic duty. Later, those Klingons fell into discord with a shady character known only as "The Albino." The Albino swore vengeance upon the first born of each of the Klingons—and kept his promise, killing the infants with a deadly virus. Among the murdered was Curzon's godson. Along with the three Klingons, Curzon took a blood oath to avenge the deaths of the children. Now reunited on DS9, the three Klingons, Kor (John Colicos), Kang (Michael Ansara), and Koloth (William Campbell), find Jadzia, who must now struggle with the moral dilemma of keeping her oath and helping kill the Albino, or sticking to her nonviolent Federation values.
"Blood Oath" is an irresistibly entertaining hour of Trek that has two levels and works splendidly on both of them. On one side, it's a lively warrior tale with lots of believable Klingon culture. The three old Klingons (reprising TOS roles from decades earlier) are always a pleasure to watch. In particular, Colicos' turn as Kor, the lighthearted of the bunch, is a joy, and has some wonderful scenes. The other side of the episode is Jadzia's dilemma, which takes on surprisingly deep and serious power—especially in a probing scene where Kira pulls Dax aside and offers some insightfully solemn words about killing. Difficult as it may seem, Fields' even-handed teleplay skillfully weaves the pulse-pounding glory of battle together with the negative connotations of violence—resulting is a very entertaining and interesting episode.
The plot moves along nicely and plausibly, the characterizations are flawless, and the stylized action scene in the final act features an impressive scope. I think Jadzia gets off a little easy in not having to make the final choice of whether or not to kill the Albino (Bill Bolender), but the wordless coda where she returns to DS9 to find a scornfully silent Sisko and Kira is beautifully done.