Star Trek: Deep Space Nine brought back older, different versions of the characters of Kang, Koloth, and Kor, all of whom had faced off against Captain Kirk (William Shatner) in Star Trek: The Original Series. Klingon mythology played a big part in DS9, particularly when Star Trek: The Next Generation's Worf (Michael Dorn) joined the cast in season 4. However, the links to Star Trek's Klingon history were established in DS9 season 2, episode 19, "Blood Oath", when it reintroduced the three TOS Klingon captains that had battled against Kirk's Enterprise.
In Star Trek: The Original Series, Kang (Michael Ansara) first appeared in "Day of the Dove" in which the manipulations of a mysterious energy being pitted him against the crew of the Enterprise. Kirk and Kang teamed up to defeat the entity, setting up better relations between the Federation and the Klingon Empire. A potential war between the Klingons and Starfleet was teased by the Organians in "Errand of Mercy", which featured Kor (John Colicos). Meanwhile, Koloth (William Campbell) was the Klingon captain who clashed with Kirk during the infamous Tribble crisis on Deep Space Station K-7. All three of these Klingon captains were reunited decades later in DS9, with some significant changes - here's how they returned and why they were changed.
What Happened When TOS’ Kor, Kang & Koloth Appeared In DS9
DS9 season 2, episode 19, "Blood Oath" reveals a tragic bond between Kor, Kang, and Koloth, which links back to a mission from decades earlier. The three Klingons led a task force that took down a brutal criminal organization led by a Klingon known only as "the Albino". In revenge, the Albino murdered Kor, Kang, and Koloth's sons with a genetically engineered virus. The TOS Klingons are revealed to have links to DS9's Jadzia Dax (Terry Farrell) via the Trill's previous host Curzon, who was godson to Kang's murdered child. All four men signed a blood oath, declaring revenge on the Albino for the deaths of their sons.
After convincing the three Klingons that she was still Dax, Jadzia joined the revenge attack, against the direct orders of her friend and commanding officer Benjamin Sisko (Avery Brooks). Koloth was killed during the attack on the Albino's compound, along with Kang who died just after killing the man responsible for his son's death, restoring his Klingon honor. Kor survived, and would return to DS9 twice more, recovering the legendary sword of Kahless with Worf and Dax, before being killed in battle during the Dominion War.
Why TOS’ Klingons Looked Like The Klingons On TNG
The differences in the Klingons between TOS and TNG have always been a contentious issue in Star Trek canon, and had to be addressed when the decision was made to bring back Kor, Kang, and Koloth. The production team briefly considered aging up the old-school Klingon make-up from TOS, but ultimately decided to make them consistent with the representation of the Klingons in TNG. The reason given to the actors was that, in the 100 years that had passed between TOS and DS9, the Klingons had developed ridges on their foreheads.
Interestingly, the idea of an albino Klingon is something that returns in Star Trek: Discovery, a show which has created its own Klingon design problems. The TOS Klingon's lack of ridged foreheads would later be revealed as the result of a failed genetic engineering experiment in Star Trek: Enterprise. Referred to as the Klingon Augment Virus, the effects had presumably been cured between Kor, Kang, and Koloth's Star Trek: The Original Series and Star Trek: Deep Space Nine, restoring their proud ridged foreheads to their full glory.