Cairn comes from a small village in the Free Marches called Wrenwith, located north of Kirkwall. He left his family to become a templar. It is implied by Nyree that he and the other templars of his order were very cruel to the mages in their care, torturing them for information, willing to cause so much pain that they had to cover the torture chamber's door with runes "to block out the screaming."
Cairn was tasked with apprehending a Saarebas who got separated from Arishok's group during his stay in Kirkwall. Along with a group of templars, Cairn arrived in Wrenwith, once his home. Upon arrival, they found the the Qunari mage Saarebas burned the village to the ground, and Cairn's sister dies in his arms. The death of his family filled his mind with the thought of vengeance and he stood out against the Chantry's orders to keep the murderer alive in order to learn the Qunari magic secrets from him. With a heavy heart, Cairn assaults the secret prison located deep beneath Kirkwall, where the man responsible for the death of his family was imprisoned. Upon the assault, however, he falls to the Qunari, unwillingly allowing his escape. When Cairn comes to, he realises the mage had escaped along with several other prisoners. To catch the man, he sets out to bring them to justice.
He meets Tallis, a Qunari elf assassin, when the two cross blades trying to apprehend the same individual who was involved with the escaped Saarebas. Cairn tells Tallis he is an acting agent of the Chantry and he is to apprehend the fugitive. Tallis catches his lie, when she puts in question the idea of the Chantry sending only one man after a group of prisoners. Cairn duels Tallis after she kills his charge, though eventually he leaves in pursuit of the escaped saarebas, warning her not to cross his path again.
Cairn next encounters Tallis when she saves his life, after he arrives at a Dalish camp while following Saarebas's phylactery, stolen from the prison. When Tallis offers cooperation in finding the rogue mage, Cairn agrees. They make a deal to duel for custody of Saarebas after they find him. The pair are joined by an elven mage, Josmael, who is attempting to locate Fina, his betrothed, kidnapped by Saarebas.
Cairn locates Saarebas in a cave where he uses Fina's blood to complete part of the Mask of Fen'Harel. During an encounter with Tallis, Cairn, and Josmael, Saarebas escapes and seals the group in the cave, along with one of his mercenaries, a reaver named Nyree.
Once Cairn and the others break out of the cave, the party, now joined by Nyree, is ambushed at an inn where the group is resting. The ambushers were hired by Saarebas, and it comes to light that it was Josmael who led them into the trap.
Cairn accuses Josmael of betrayal and attempts to kill him, but is prevented by Nyree. Nyree reveals that she witnessed Cairn enter Saarebas's cell before the escape. Cairn, forced to reveal that he inadvertently helped the mage escape, explains that he had gone to kill Saarebas to avenge the death of his family, but instead accidentally freed Saarebas from his bond and was overpowered. The group sympathizes with Cairn and then sets out for the final confrontation with Saarebas.
While lying in wait for Saarebas at Sundermount, Cairn has a conversation with Tallis about their purposes for hunting the mage. He insists that Saarebas needs to die, and encourages Tallis to abandon the Qun and become a renegade, a Tal-Vashoth. Tallis rejects the idea, but does not resist when Cairn kisses her.
After Saarebas arrives, Fina betrays Josmael by alerting Saarebas to the group's presence. While the group battles, Josmael is forced to stab Fina. Using blood magic, Saarebas completes the ritual, which tears open the Veil. Before the power of the Fade can be unleashed, Tallis shatters the mask, sealing the Veil shut.
Cairn confronts Saarebas, who shows no remorse over killing Cairn's family. Saarebas overpowers Cairn, then battles Tallis. When Saarebas unleashes a fatal bolt of lightning, the templar throws himself in its path, saving Tallis's life. To avenge Cairn's death, Tallis fulfills Cairn's wish and kills Saarebas.Tallis, Nyree, and Josmael bury Cairn the following morning atop Sundermount.
It is not possible for Cairn's home village to be located north of Kirkwall, as north of the city there is only land, and Saarebas supposedly beached after the Qunari ship was sunk at sea in the south.