Raised by parents with a staunchly 'Altmer Supremacy' views, she left home at a young (for an elf) age to join the Thalmor. With them, she developed into a highly skilled battle mage, known for her successes against rebel groups in Valenwood and during the Great War. It was with this reputation that she was sent to Skyrim to aid the weakening Imperial forces against the Stormcloak Uprising (at least, that was the official story; their actual purpose was to perpetuate discord because a weakened empire is better for the Third Almeri Dominion, and the Thalmor sensed that the Imperials were close to losing and therefore the uprising close to being over.) But it was in Skyrim that events unravelled for her; she was unused to fighting against 'soldiers' who have little sense of honour or rules of war, and as such was no real match for the Stormcloaks... After her party's failed assault on a Stormcloak camp and during their subsequent attempt to retreat back to the Thalmor Embassy (having abandoned the Imperial soldiers along the way), the Stormcloaks caught up with them. Most of the party were slaughtered. She and a handful of other high ranking members were captured as prisoners and taken to Windhelm.

In Windhelm, they were lengthily and brutally tortured, partly for information and partly from pure hatred. Some did not survive the ordeal; although she never found out of the official numbers, of seven prisoners it is rumoured that only two emerged alive, she and one other. Not that she emerged very alive; the torture had rendered her unable to use her magic and had wiped her mind clean. Her memories of her childhood and former life, her activities with the Thalmor, even the events that had led to her capture... All gone, or at least heavily suppressed and unable to be accessed for the meantime. It was in this state that the Stormcloaks had 'reprogrammed' her, as it were, to believe she was a Stormcloak. Then they planned to have her sent back to the Thalmor Embassy, who would accept her back thinking she had managed to escape ... but this plan was not to come to fruition. The cart she was on (with intent to escort her as far as Ivarstead, close to where she had been captured in the first place) was caught in an Imperial ambush near Darkwater Pass. Some of the soldiers she had known previously recognised her and attempted to save her, but she did not have enough sense of who she was to co-operate. She insisted she was a Stormcloak and gave the only name she could remember, the name her Stormcloak torturers had mocked her with: Sigrun, or 'secret victory'. Against all odds, it had to be assumed that she had defected, and it was under this belief that she was sent to Helgen, with regret, to be executed.

Even that did not manage to go to plan. The journey was long, and without the continued torture and feeding with the mind suppressing drug, Sigrun began to come out of her stupor. Not by much - she still had no real idea of who she was and her only recollection of how she ended up on a cart heading for Helgen ended at 'captured by Imperials', she had no concept of why she was with the Stormcloaks in the first place. But she was perhaps a little more herself than she had been a few days ago. However, she still refused her final chance of reprieve when she continued to insist she was not with the Thalmor. Her execution was bizarrely and abruptly called to a halt with the appearance of a dragon, the first seen in Skyrim for millennia. She did not find out until much later that this perhaps was not a coincidence and that her presence at Helgen may have had something to do with it after all... Never mind for now. Still convinced she was a Stormcloak, she fled Helgen with Ralof, although during her journey through the rapidly falling keep she started to remember brief flashes, enough to realise that the Stormcloaks as a group are not her friends, although Ralof himself seemed oblivious to who she actually was. Nonetheless, she assassinated the Stormcloaks she could and separated from Ralof as soon as possible after emerging from Helgen. She ran until she reached Ivarstead, where she at last deemed herself safe enough to rest and recover. Her magic was beginning to re-emerge although it was still rather weak, but she could remember (more or less) what had happened in recent events.

And so, she formulated a plan. She was in a unique position; she could remember who she was but had no obligation to return. The Thalmor would assume her either a traitor or dead, whether by execution or by the dragon attack. If anything, it would be difficult for her to return to them. The Stormcloaks may still be looking for her; Ralof knew her assumed name (and she was still unable to remember her actual one) and was going to take word back to Windhelm. For the first time, it occurred to her that she was still dangerously close to Windhelm and might not be safe for long. She had to find somewhere to go, but she had no where. The best option she could consider was to head north to Winterhold; there was a neutral college of mages there if she recalled correctly, and the Nords were already scared because they believed the mages responsible for the near destruction of the town. They would likely not touch her there. Whilst there, she may be able to recover her magic skills and decide on a more long term plan. She planned a tortuous route that avoided Windhelm and meant she would be able to seek shelter in the Nightingate Inn. And so, by the cover of darkness, she set off. The journey was long and hard, but she made it. She was safe with the College of Winterhold.

Whilst there, she began to return to her normal self. Her magic skills were improving with the continued assistance of the senior mages, and her involvement with College business was seeing her rise through the ranks. She had found a mentor in the Arch-Mage, and whilst she was still rather cagey about her past (not least because she still couldn't remember it all) she started to form friendships with her fellow students, even briefly forgetting her formerly staunch beliefs in Elven and particularly Altmer Supremacy. For example, she helped them out and even began to ask Onmund - a Nord - to join her on her outings. The only negative aspect of the College was the Thalmor presence in the form of Ancano. However, he did not seem to recognise her and has no reason to believe she is suspicious, especially given that she was still not using her real name (having given up trying to recall it.) For a while, life was good and Sigrun found the research needed into the mysterious Saarthal orb fascinating ... then events took a turn for the worse. Ancano suggested that he may after all have an inkling of Sigrun's background and, even more disturbingly, hinted that her capture may not have been an accident after all... Then he decided to harness the Eye of Magnus, killing Sigrun's mentor Savos Aren in the process. He was eventually defeated, but not without great loss for Sigrun. Her mentor was dead; she no longer felt safe at the College; she had too many questions about her capture to remain of peaceful mind; and she finally and fully understood the destructive power the Thalmor wield.

In light of these revelations, she chose a curious path: she decided to seek out the Imperial Legion. Onmund, perhaps her closest friend, was horrified and refused to join her in her quest; she attempted to persuade him, but as she could not yet reveal her full plan she was forced to part from him and leave him behind. It was commonly assumed she earned her suffix name 'the Betrayer' for other reasons, but it was actually given to her at this time by Onmund. For a long time she blundered across the tundra towards Solitude, where she spun a lie to cover her long absence. Unsurprisingly to Sigrun, she was asked to re-prove herself, which she did up until the point of retrieving the Jagged Crown with the Imperials. Then she performed an even more curious move: she delivered it not to General Tullius, but to Ulfric Stormcloak instead. They were surprised to see the return of Sigrun; they were even more surprised that she was in near-full recollection of her past and still chose to come to them. She explained that she saw the evil of the Thalmor for herself and that they need to be put down; as she saw it, the Stormcloaks had the greater chance of winning and a united Skyrim would have a better chance of defeating the Thalmor than a warring Skyrim, or a Skyrim still aligned with them. When asked why she did not just return to the Summerset Isles, she answers with the truth: there was only death for her there. Her parents also betrayed the Thalmor at the eleventh hour, the reason for her sacrifice to the Stormcloaks in the first place. She was convinced the Imperials were intending to kill her in the near future.

And so Sigrun set about liberating Skyrim in the name of Stormcloaks. She wished to return to the College and tell Onmund what was happening, but found she felt too scared to do so. Instead, she wore an Amulet of Mara in his honour, continued to use his derogatory name in rememberance and thought of little else beyond the up coming war and Onmund. Before too long, though, she found herself distracted by a greater cause: she began to learn more about what happened at Helgen and how exactly she was involved... Temporarily forgetting the war, she ventured down her destined path long enough to discover her true Dragonborn nature and that the Thalmor would have to be infiltrated. Wanting to avoid this if at all possible, she returned to the war efforts and finally completed them, although the Thalmor still held strong in the embassy, albeit it in a state of near permanent siege. She infiltrated them as it was promised she would do if needed, and continued down her destined path - the reforging of a strong Skyrim was not her concern, merely providing the Stormcloaks the chance to do so. She did, however, continue to massacre any Imperials or Thalmor that she encountered. Not much further down the line, she learned that she needed to return to the College in order to harness a knowledge held there. The idea of seeing Onmund once again after everything that had transpired filled her with nerves.

Yet she set off, going by foot almost in pilgrimage. Her return was unexpected – Tolfdir had been running the College in her absence, assuming it was a permanent state. Her former friendships lay in tatters for the most part, even Onmund mistrusting her, even when she told him of everything. She discovered that the knowledge she desired lay down a dangerous path, and Onmund loyally agreed to accompany her down it, although they never returned to their former state. He did continue to provide her with the support she needed, however, and together they maneuvered her to a good position from which to defeat Alduin. Once she left him to journey to Skuldafn and Sovngarde, he warned her that he would not be waiting for her return. And, true to her word, he was not. She returned from Sovngarde the slayer of the World Eater Alduin, Hero of Skyrim, and yet all alone. She returned to her house in Windhelm, heartbroken. On the rebound, she married a former soldier she had sought advice from in her early days as a soldier. Then she left him everything she owned, and returned to the College.

She entered unseen, stole to the top of tower and plunged to her death from there, drowning in the Sea of Ghosts. No one even knew that Sigrun the Betrayer was dead.