Star Trek: Deep Space Nine, season 2, episode 19
Blood Oath
Summary
Jadzia Dax honors an oath made by Curzon Dax to three Klingons (Kor, Koloth, and Kang), and goes with them on a crusade against their sworn enemy "the Albino" who murdered their firstborn children as revenge for stopping his raid on a Klingon colony.
Summary from Wikipedia, CC BY-SA 4.0.
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