His son would later recall that Tain never grew tired of repeating to him "I should have killed your mother before you were born. You have always been a weakness I can't afford." (DS9: "In Purgatory's Shadow") Tain would also discipline his son, who had a strong case of claustrophobia, by locking him in a closet. In contrast to this, Tain spent a day in the country with his son when he was four, their "only day" as Garak would later recall. Garak impressed his father by mounting a riding hound despite falling off numerous times. As he limped home, his father held Garak's hand and would later recall that he was very proud of him that day. (DS9: "In Purgatory's Shadow", "Afterimage")
Tain became head of the Obsidian Order in 2348, a position he held for twenty years until his retirement in 2368, and was the only head of the Order to live long enough to do so. Tain trusted no one, with the exception of his housekeeper, Mila. He was known for ruthlessness, and many said that he lacked a heart. Tain was also Garak's immediate superior, whom he trained and molded into a mirror image of himself. Nevertheless, Tain was directly responsible for exiling Garak after being betrayed by him in some way.
In 2370, during his retirement, Tain lived on the Arawath Colony where he was visited by Julian Bashir, who was seeking information on Garak's cranial implant so that it could be removed. Tain claimed to be only helping Bashir so that Garak could live a long, miserable life. (DS9: "The Wire")
The following year, Tain attempted to stage a comeback by destroying the Founders' homeworld with a combined fleet of Obsidian Order and Tal Shiar ships. His plan was compromised by a changeling infiltrator, and the resulting Battle of the Omarion Nebula was a complete failure. The entire combined fleet was destroyed by the Jem'Hadar. Tain was assumed to have perished when his warbird exploded, however he was actually captured by the Dominion and detained at Internment Camp 371. (DS9: "Improbable Cause", "The Die is Cast")
In 2373, Tain managed to modify the camp barracks' redundant life support system to send a subspace signal to Garak, indicating he was alive. By the time Garak reached him though, he was dying of unspecified heart trouble. On his deathbed, Tain asked Garak to escape, so that he could seek vengeance on the Dominion for what it had done to him. Garak agreed, but only if Tain asked him as his father. Tain died in the presence of Garak and Bashir after acknowledging that Garak was his son, his last words were to Garak, "I was very proud of you – that day." (DS9: "In Purgatory's Shadow")
A younger Tain appeared in the Stargazer novel Oblivion, where he is pitted against a young Captain Jean-Luc Picard during Picard's first meeting with Guinan. During this mission, both Picard and Tain are looking for a friend of Picard's who claims to have vital information about a potential threat, but it is eventually revealed that the friend has betrayed Picard and actually has no information. At the end of the novel, Picard and Guinan are transported onto the USS Stargazer, leaving Tain to drift through space on an abandoned, badly damaged spaceship until he is recovered.
The Star Trek: Deep Space Nine novel A Stitch in Time reveals that Mila was Garak's mother and that Tain hid his paternity by having Mila's brother Tolan raise Garak as his son. Later, it was Tain who sponsored Garak's entry into the elite training academy for Obsidian Order recruits.
His mirror universe counterpart appears in the novel Rise Like Lions, in which is depicted as holding the position of head of the Obsidian Order until his death in 2378. He refers to Elim Garak as his illegitimate son.