This is not just the backstory, but an account of her journey. I may have missed the mark involving some of the lore, so please forgive me.
Race: Dunmer (female)
Follower of Mephala
Weapons: Ebony Blade
Spells: Fire Destruction, Fenzy/Calm, Muffle
Armor: Ebony Mail, Ebony Helmet (fortify destruction), Ebony Gauntlets (fortify 2 handed), Ebony Boots (fortify sneak), Ring/Necklace (fortify destruction).
Skills: 2 Handed, Illusion, Sneak, Heavy Armor, Enchanting
Quests/Factions: Dark Brotherhood, Main Quest, Dawnguard (vampire), Oblivion Walker (achievement)
Achlys grew up on the island of Solstheim, on an ash yam farm with her parents. She had no idea they were secretly Morag Tong assassins, working for one of the last Grandmasters of Morrowwind. She would find out after the couple did not return from a mission. Her parents had received word the Sithis worshiping Dark Brotherhood had sent an assassin to interfere with an inter-house rivalry. They quickly went to stop their return to Skyrim, but were betrayed by an unknown, and never returned.
Now orphaned and alone, Achlys ran. She was found by the Grandmaster. He took her in and raise her in secret, fearing the Dark Brotherhood would return for her. She trained as a Morag Tong along the rest of the members, but her past was kept from them for her protection. No one ever found the betrayers. He taught her the ways of Mephala, and how to kill in secret, cover it up, and make it seem to be someone else. After decades of training, she would be given her first Writ, though she did not know it would be the only one she ever received.
Upon reading it, she knew straight away why the Grandmaster had chosen her for the job. It read, "Travel to Skyrim. Mephala will contact you through two mediums. One will be in the form of a young boy. The second will be through the Dark Brotherhood. While the second quest is the ultimate mark, the first is more important. The boy knows where the Ebony Blade is hidden. Get it back, and use it to destroy the Dark Brotherhood from the inside out. Remember... Mephala will guide your hand." Posing as a refugee, she immediately set out for Skyrim, never expecting to return.
Her ship landed in Windhelm, where her kind are harassed and segregated to one side of the city to live poorly. She wandered the city for a few days listening for any leads. None presented themselves, and she had no where to go. Eventually, after a few years had passed Achlys grew resentful of the treatment she endured as a Dunmer. It took all she had not to act on her impulses, and use her training on the people degrading her race. Finally she snapped. She began using her illusion magicka, taught to her by the Grandmaster, to turn the city's people against each other. At one point, she made the decision to abandon her mission and act on her own. She planned to frame Ulfric Stormcloak. She followed him on one of his raids. Little did she know, he had others hunting him as well. Just before making her move, Ulfric was captured by the Imperial Legion. Achlys tried leaving unseen, but found herself paralyzed, as if under a spell. She, in turn, was captured. It seemed she would share the fate of her prey.
They prisoners were taken to Helgen. Just as the axe was to fall on her head, a dragon fell from the sky. Quickly, Achlys fled with the help of the others. After navigating an underground escape route, Achlys is led to Riverwood. There she learns a little more about the dragons, gathers supplies, and heads to Whiterun at the request of her suppliers. She arrives early in the morning. After gaining entry to the city, she makes her way to Dragonsreach to inform the Jarl of the dragon's rising. He fears for his people and sends guards to Riverwood, and sends Achlys (at the request of the court mage) to retrieve a stone tablet that he thinks may have some answers. initially, Achlys refuses to get involved, until she meets the Jarl's son. Achlys can sense Mephala in his words, and remembers the words of her Writ from years ago. She decides to go along with the Jarl's wish so she may not raise suspicion while observing the boy.
Achlys is sent to a Barrow to find the stone. Upon finding it and returning to the Jarl's court mage, she is asked one more favor. She is now sent with a group of soldiers to see about a dragon near Whiterun. As she has no further information regarding the boy, she accepts, in order to gain the favor of the Jarl. As he feared, a dragon attacks her party. After dispatching the the fiery threat, she learns something about herself. The soul of the dragon is absorbed into herself, granting her its power. She returns to the Jarl and tells of the dragons defeat and her ability. He tells her of the Dragonborn, and the way of the voice. If she wanted to know more she would need to visit the Graybeards. For a moment she forgot about her search for Mephala, and sought a place where she could come to terms with the days events. She wandered into the local tavern for rest and refreshment. While there she overhears the barkeep speak of a rumor involing the Jarl's son. Finally a lead!
Suddenly shaken from her distraction, Achlys hurries back to Dragonsreach to ask the Jarl about his son, and the rumors. He doesn't offer much. As a result Achlys decides to approach the child herself. He speaks of a "Whispering Door" and a voice asking him for secrets, and telling him some as well! This must be the child who was to lead Achlys to Mephala. After searching, she finds the door. It is locked, but the voice still speaks. Alas, Mephala! After a few formalities, Mephala instructs Achlys to open the door and claim the power behind it. The Ebony Blade she was sent for all those years ago! With the help of the child, she claims the key from the court mage. Upon entering the basement, and claiming the blade, Mephala explains the blade must first claim the life of one close to Achlys. Achlys was officially back on her mission. She charged the blade and set forth looking for the Dark Brotherhood.
After wandering Skyrim for sometime, further charging the Ebony Blade, Achlys found herself, once again, in Windhelm. She was searching out another young boy who was rumored to be attempting contact with the Dark Brotherhood. She finds him to be a fellow orphan. He speaks of a horrible person he wants killed. As this was the only lead towards encountering the Dark Brotherhood, she set out for Riften. After arriving, and questioning the inhabitants, she finds the woman and kills her. Achlys then hangs around town for a few sleepless days, learning more about the Dark Brotherhood from some of the members of the Thieves Guild. She also hears about a group called the Dawnguard from a passing guard. This group interests her, as her hunger for power grew as the Ebony Blade was charged. She had heard of the Vampire Lords long ago from the Grandmaster. That night she finally was able to sleep, only to awaken to a surprise!
Achlys wakes up feeling hazy, as if drugged. She sees a woman in the corner who identifies herself as the leader of the Dark Brotherhood. Her Dunmer blood boils as she fights the urge to kill her on the spot, but she remembers again the Writ. She complies with the woman, knowing this will lead to another meeting with Mephala, who will tell her when the time has come. After traveling to their sanctuary she spends some time learning their ways, studying their habits and system, and their goals. After some time Mephala again shows a sign. The Nightmother's arrival is a good omen for Achlys, as she is believed by some to be a vessel of Mephala. The Nightmother eventually speaks to Achlys, naming her the Listener. Immediately, she sees the big picture. The leader of the organization is already threatened, and begins tearing the group apart. All Achlys need do now is carry out Mephala's bidding through her role as the Listener, and watch the Dark Brotherhood burn from within!
After completing multiple requests from the Dark Brotherhood, a contract comes up that even Achlys is excited to be a part of. Kill the emperor. It would not be easy, and required multiple steps. Achlys did as told by the Nightmother, but found herself betrayed my none other than the leader of the Dark Brotherhood. "How symbolic", smiled Achlys to herself. "They wish to kill me by interfering in my contract. The same way they killed my parents." In a calculated fury, Achlys fought her way back to the sanctuary only to find it burned, and most everyone already dead. That is except for the leader, who had betrayed her whole family in her jealousy. Mephala had finally blessed Achlys with the chance she had been waiting for all those years. She killed the leader with the Ebony Blade fulfilling her revenge, and completing her Writ. The remaining members moved and asked Achlys to come. They saw at once she would not, as she gazed back at them with silence and a fire in her eyes. they now understood what it was to be lost and alone.
Achlys left feeling empowered. She set forth to collect the Daedric artifacts from their Princes to use as her own. Now free to do Mephala's bidding, she would seek out Harkon. Time passed and she eventually collected all of the remaining Daedric artifacts. By now many people across Skyrim had heard the whispers of a dark figure bringing misery to all that opposed her. Now free from her Writ, she sought only to build her powers through lies, secrets, and evil plots against any Mephala ordered. This included even some of the Daedric princes as well, as Mephala had many enemies. Her first Daedric target would be the followers of Mephala's rival Molag Bal, whom she had encountered on one of her Daedric quests. This led her to seek out the Dawnguard to find Harkon, where she would gain the powers of the Vampire Lord. Eventually, she killed their leader. using the powers he had given her to turn his family against him.
Now with her hunger for power growing greater, Achlys sought to destroy any who may be stronger. She had once heard of Alduin while helping the Jarl of Whiterun. It seemed like a lifetime ago now, but she remembered his words about the way of the voice. Her journey in Skyrim had come full circle. Alduin the World Eater, she decided, would meet his death. Once again, Achlys would use the friends and powers against her foe.