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 to 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.
The costume worn by Nordling as Tahna was later sold off in the It's A Wrap! sale and auction for US$113.50. It consisted of a green smock style top with puckered fabric, ribbing around the shoulders and a zip up the back, and matching pants. 
In the original script for "Past Prologue" Kira and Tahna were to be lovers, but Executive Producer Michael Piller decided that this was a television cliché. The original version also had Tahna turning his back on terrorism and attempt to forge peace with the Cardassians, before being killed by his own people. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p 41) The final script describes him in his first appearance as, "bloodied and weak." 
In their review of the episode, authors Mark Jones and Lance Parkin were critical of Tahna's plan to destroy the wormhole, writing, "The major problem is Tahna's ploy - if the bomb is powerful enough to irradiate the system, won't Bajor be affected also?" (Beyond the Final Frontier, p.183)
In the Star Trek: 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's mirror universe counterpart appears in the Star Trek: Mirror Universe short-story "Saturn's Children" set in 2375. A member of the Terran Rebellion and first officer of the Defiant-class warship Terra Victor (β) under the command of Captain Kasidy Yates (β), he is later killed during a battle with the IKS Ya'Vang (β).
The character also has two cards in the Star Trek Customizable Card Game, one of which describes him as the "Voice of the Kohn-ma".