Kahn was a Trill symbiont, a symbiotic lifeform that existed with the humanoid Trill as a joined species. Each joining of Kahn with a host created a new, unique individual, but each individual also carried the memories of the previous hosts.
History
Kahn was born on Trill, in the Caves of Mak'ala like all Trill symbionts. (DS9: "Equilibrium")
By 2285, the Kahn symbiont was joined to Nilani, who was married to joined Trill Torias Dax. In that year, Torias was killed in a shuttle accident, leaving Nilani heartbroken.
By 2373, the Kahn symbiont had been joined to Lenara. She visited Deep Space 9 on a scientific mission, working to create an artificial wormhole, and met Kahn's ex-husband, Dax, now Jadzia Dax. The Kahn symbiont considered reassociation, but that would put it in danger of being expelled from Trill, never allowed to return. After temporarily rejoining, Jadzia Dax was ready to take that risk but Lenara Kahn decided it would be best to stay apart. (DS9: "Rejoined")
Appendices
Apocrypha
Some time between 2373 and the early 25th century the Kahn symbiont was joined to its seventh host, Damar Kahn, who like many of the previous Kahn hosts is a scientist, who in this lifetime studies the effects of supernovas. He is found on Starbase 114 in Star Trek Online. During the episode "Heading Out" you are to pick him up from Starbase 114 and deliver him to Starbase 39-Sierra where he will board the U.S.S. Tucker for the Hobus system.