Previously a member of the Bajoran Resistance during the Occupation of Bajor, Tahna later joined the terrorist organization Kohn-Ma to continue his fight against the Cardassians and his fuel his dislike of the Federation's involvement in Bajoran affairs.
In 2369, while escaping from Cardassian Gul Danar, Tahna approached Commander Benjamin Sisko with a request for political asylum. Kira Nerys, who was a member of the Resistance with Los, asked Sisko to grant his request, which he did. Upon learning of this, Danar demanded he be turned over to them.
After the asylum was granted, Kira brought his case before the Ministers' Court in an attempt to grant him amnesty for his past criminal acts. They agreed to convene a hearing about the request, and two members promised their immediate support, as he could help with the rebuilding of Bajor. With the expected support of a third member, the hearing would have been a mere formality and amnesty would have been granted. That, however was not to be.
Tahna believed that Bajor would have no strategic value if the wormhole was destroyed. He used his asylum as an opportunity to destroy the wormhole. The Klingon sisters Lursa and B'Etor arrived on the station and sold Tahna a cylinder of bilitrium, an ingredient needed for a high-powered bomb. Tahna equipped the USS Yangtzee Kiang with bilitrium. His plot failed, however, when Kira steered the Yangtzee Kiang straight into the wormhole, and released the weapon, which exploded harmlessly in the Gamma Quadrant, on the other side of the wormhole. Tahna surrendered and was later taken into custody aboard Deep Space 9. (DS9: "Past Prologue")
Tahna Los was played by actor Jeffrey Nordling.
In the Deep Space Nine novel A Stitch in Time, Tahna is arrested again and placed in a Bajoran prison, where he remains until at least 2375. He later informs several other Kohn-Ma members, including Tir Ramara, of Elim Garak's continued presence on Deep Space 9. Tahna suspects Garak of being involved in his capture.
Tahna Los also appears in the Star Trek Customizable Card Game.