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.
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 was ready to take that risk but Lenara decided it would be best to stay apart. (DS9: "Rejoined")
In the non-canon online game Star Trek Online, some time between 2373 and the early 25th century the Kahn symbiont was joined to its seventh host, Damar Kahn. Damar, who like many of the previous Kahn hosts was a scientist, studied the effects of supernovas. He is found on Starbase 114 in the game. 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 USS Tucker for the Hobus system.