Martin was born an illegitimate child to Uriel Septim VII and an unknown mistress. Uriel could not raise the child, and trusted Jauffre with spiriting him away as an infant. Uriel still asked Jauffre for updates on Martin's whereabouts, but never met with the boy anytime after he was given up. Martin was unaware of any of this for most of his life.
In his youth, he grew up believing that his father was a farmer and his mother had died giving birth to him. Early on in his life, he discovered the seductiveness of Daedric magic, practicing it and using it on a daily basis. Martin joined the Daedric Cult of Sanguine. During that time, Martin briefly came into possession of the Sanguine Rose.
Martin notes that he and his friends got in over their heads and some of his friends died in a disastrous accident. Later, a priest managed to convert Martin to the worship of the Nine Divines. He became a priest at the Chapel of Akatosh in Kvatch.
Following the death of his father and half-brothers by Mythic Dawn assassins, Martin is the only remaining Septim. Unaware that he is the sole remaining heir to the Septim throne, Martin is sought out by the Hero of Kvatch. Upon being found and informed of his true heritage, he helps to end the Daedra invasion of the mortal world.
Soon after Martin's discovery, the Amulet of Kings is stolen by members of the Mythic Dawn, a Daedra cult based within Cyrodiil, who were also responsible for the invasion. To safeguard Martin, Jauffre, grandmaster of the Blades, takes him to the fortress of the Blades, known as Cloud Ruler Temple, where he also offers to make the Hero of Kvatch a member of the Blades.
A failed attempt by the Hero to recover the Amulet of Kings from Mankar Camoran yields the Mysterium Xarxes, the holy book of the Mythic Dawn cult and a tome of massive dark power. However, it is slow, painstaking work, taking much research.
Eventually, Martin determines that the book could be used to open a portal to Camoran's Paradise and that four other items are needed to do this. These include any Daedric artifact, the blood of one of the Nine Divines (obtained from the armor of Tiber Septim, in which he had fought and bled before his Apotheosis), a great welkynd stone, and a great sigil stone.
The Hero obtains the first three objects on his own but the last may be obtained only by allowing the Mythic Dawn to open a Great Gate (which requires three minor Oblivion gates to open) outside of Bruma. This could potentially cause the destruction of Bruma. Martin dons the armor of Tiber Septim and leads a Defense of Bruma, deliberately allowing the opening of the gate and buying the Hero time to secure the stone.
The four objects in hand, Martin binds himself temporarily to the Xarxes and opens a portal to Paradise. The Hero of Kvatch enters, kills Camoran, and returns with the Amulet of Kings, presenting it to Martin.
During this raid, Martin sends the Blade Baurus to the Imperial Palace to formally present his claim to the throne to Chancellor Ocato of Firsthold, who, with the Elder Council, considers and accepts the claim.
Arriving at the Imperial Palace with the Hero and Jauffre, Martin is about to be formally recognized as Emperor by Ocato when a massive Daedric invasion erupts within the Imperial City. Martin, Ocato, the Hero, the Blades, and numerous Imperial Guards fight their way to the Temple of the One District to relight the Dragonfires.
However, they are too late; Mehrunes Dagon himself has entered Tamriel. Lighting the Dragonfires is pointless, as they had merely maintained the barriers between Mundus and Oblivion.
A despairing Martin turns to the Hero and a brief conversation ensues, during which the Hero asks, "What about the Amulet of Kings?" Martin is inspired by the question and orders the champion to lead him to the Temple.
Upon their arrival, the Hero and Martin bid each other farewell. Martin then shatters the Amulet of Kings, binding his blood with that of the kings and gods held within the central gem of the amulet, in order to become the avatar of Akatosh; he takes the form of a giant dragon wreathed in flame. Dagon smashes the Temple Dome as Martin transforms. The Dragon then destroys Mehrunes Dagon's physical body, banishing him back to the Wastes of Oblivion, but not without sustaining a mortal wound itself. After the fight ends, the Dragon arches its neck, roars into the sky and is petrified, becoming a statue.
Soon after this event, Ocato of Firsthold states that Martin would be remembered as the last and greatest Septim, and goes on to say that Martin would have been a great Emperor. As Martin died childless, like all of his brothers, the Line of the Septims is completely defunct. The nature of Martin's sacrifice ensures that an Emperor is no longer necessary to maintain the barriers between the worlds; however, the fate of Martin himself remains unknown.
The ending of this bloodline fulfills the prophecy of the Elder Scrolls that foretells the return of Alduin, and the coming of the Last Dragonborn which continues in the events of The Elder Scrolls V: Skyrim.
- While Martin's mother is unknown, Caius Cosades suspected that it was a girl named Gemile.
- Martin falls in along with many other characters that Sean Bean has portrayed, due to the fact that they are killed during a climactic scene.
- In Skyrim, the Thalmor claim they ended the Oblivion Crisis within the Aldmeri Dominion. The Thalmor made this claim to the people of the Summerset Isles after the Oblivion Crisis in order to gain the favor of the desperate people.
- When he is first met in Kvatch, his name is shown as "Brother Martin." When he is brought to Cloud Ruler Temple, he is referred to as "Martin." After returning from Paradise at the end of the Main Quest, he is referred to as "Martin Septim."
- During the quest "The Mind of Madness," Sheogorath says he considers Martin Septim one of the greatest Septims that has ever ruled, if not the greatest. Although, he says the only reason for that was Martin turning into a Dragon God, which was "hardly sporting".
- Martin's inventory, weapons and clothing change dramatically throughout the questline. When the Hero meets him in Kvatch, he wears a grey robe and buckled shoes and wields a dagger of sparks. After returning from Miscarcand with the great welkynd stone, he wields a silver longsword of frost and is clad in Emperor's armor. And upon triumphing over Mankar Camoran in Paradise, he will be wearing the Emperor's robe and Emperor's shoes