Not much is known about Aeirillen’s origins and who her parents were. Found wandering on the edge of Lothlorien as a small silvan elf child in the year 470 of the Third Age she was just 14 at the time. Taken under protection by the elves of the Golden Wood it was quickly discovered she was cunning and agile and with time she also showed great talent in marksmanship and dual wield. Therefore when she reached her adulthood in year 556 she decided that becoming part of the elven military force would be a path worth pursuing - a decision based on the urge to protect other young elves from her fate as an orphan.
She made her way to Imladris to pursue her further training, during which she befriended a young Noldor elf named Veras and together they spent years preparing for their lives as soldiers.

In the year 861 of the Third Age, when the city of Annuminas started to fall into ruin Veras, and five other elves were sent to Evendim under Aeirillen’s command for a recon mission. Main objective was to gather the intel on the war situation in the North and report back to Imladris with whatever information gathered. During that mission they established the fate of the Annuminas scepter which at the time had been moved to Fornost. As they were preparing to return to Rivendell, Veras had been captured by a large group of orcs also scouting that area. 

Aeirillen’s judgment was clouded and desperate to save her best friend she gave an order to attack the orc camp. Their force, however, was too great and her entire commando, including Veras was brutally slain and she herself was badly wounded, saved only by the currents of Evendim lake who took her unconscious body to the other shore. Once the orcs departed she returned to bury her friends. When she found Veras he was still barely alive but not knowing how to heal him she did not manage to save his life and he passed away in great pain. She buried him with the rest, along with a necklace he once gave her as a symbol of their friendship and loyalty to their values.
  Heartbroken and ashamed she made her way back to Imladris to report the information they gathered and to confess the mistakes she made as the leader of the group. She stepped down from a position of a commanding officer, abandoning the military feeling unworthy of pursuing that path any longer. For several centuries she focused on learning the arts of elven healing, hoping that saving others lives would make up for her past mistakes and also to never feel as powerless as she felt when Veras’ life was slipping away before her eyes.

 Although forgiven by her kin she could not shake the guilt and grief after the loss of those under her command and she decided to leave the valley feeling she will never feel at home among them again and she was not seen for a very long time.
In the year 1636 during the Great Plague she could have been seen wandering the northern Rhovanion providing as much medical aid as she could to the affected. She chose that region mostly because of the medicine being undeveloped there in comparison to other lands. Once the plague ended she vanished again in the forest of (then) Greenwood.

 It was there when she decided that staying in Middle-Earth was too painful for her and made a decision to sail to Valinor, however as soon as the decision was made a voice of Lady Galadriel rang through her mind with a promise of a new purpose if she’d only decide to take this new path. Although hesitant to return to her kin, Aeirillen saw a glimpse of hope for her fate in Middle Earth and a chance for redemption. She returned to Lothlorien in the year 1995 of the Third Age where she resumed her military training as instructed by the Lady of the Golden Wood.
In TA 2063, when Sauron abandoned Dol Guldur and the era of Watchful Peace began, Lady Galadriel formed a special division of elves. A minor yet very skilled military group operating in secret in service of the Lady. Their task was to be the eyes and ears of Lothlorien in Middle-Earth and, if needed, to take care of more delicate matters involving diplomacy or sometimes even just brutal force. Each member made an oath to serve Galadriel therefore binding their fate with her until the end of time. And so did Aeirillen from now on wandering the lands in search for the signs of the enemy, bound to act in service to the Golden Wood and follow the commands of her Queen.
During her travels she often found herself in the company of men, a race she once considered weak and unworthy grew on her greatly and with time she started to enjoy the simplicity and love for life they offered.

In TA 3018 her path was bound with a mercenary group calling themselves The Black Steel and for months they traveled together involved in all sorts of dealing around Eriador focused on maintaining peace and safety across the land. Their time together had to come to an end however, as the duty to Lady Galadriel summoned her once more and Aeirillen’s skills were needed elsewhere. The elven force has reunited and taking the name of Blades of Lothlorien they embarked on a secret quest.

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Successful in their task the Blades made their way back to Lothlorien and disbanded. They all headed their own way waiting to be summoned again to join their Queen once and join her in her journey to Valinor when the time comes.

The success did not ease the guilt and remorse Aeirillen still felt because of her past mistakes and she could not recover from the loss of her best friend. She left the Golden Wood heading for Eriador, hoping to see the Valley of Imladris once again to face the memories once made there. While on her way to the Gap of Rohan she came across a group formed by newly crowned King Elessar heading to reclaim the city of Annuminas, and she joined them in their travels since their paths aligned.
During their travels, as they passed through the Lich Buffs they investigated the presence of the wights there, an adventure Aeirillen would pay for dearly. Accidentally touching the necromantic stone she found herself under the influence of a curse. The curse would torment her mind with the visions of those she had lost and tempt her to use the powers of the stone in order to bring them back. The longer she resisted the more the corruption fed on her fea (spirit), depriving her of all the elven skills and powers, weakening her greatly.
Furthermore, the curse also affected her ability to hear the summons of the Valar which would suggest she might not be able to ender the undying lands when her time comes and that idea terrified her beyond measure.
Having to rely on the protection of the group from now on she made it to Imladris where it was revealed that the only way to rid of the curse would be for her to let go of clinging to her past. And thus she embarked on another adventure under the banner of The Silent Pilgrims, heading for Evendim.

Upon arriving in Evendim she and her companions headed to the place of burial of her former commando to find it drowning in the stench of death and necromantic power once more. Aeirillen’s guilt combined with the power of the stone seemed to have awakened the spirits of the fallen elves (including Veras) and corrupted them into rogue spirits. She would not win over the horror she faced that day if not for the assistance and support of her new friends. Together they managed to defeat the spirits and put them to rest and Aeirillen finally learned to let them go therefore overcoming the power of the curse. 
Shortly after those events the group joined forces with the Dunedain and a small group of elves they met in their travels and together they reclaimed the city in the name of King Elessar.
With the curse lifted and finally making peace with her past she realized she was not ready to depart from Middle-Earth yet and she’d wish to stay there for as long as possible in order to help the men rebuild their kingdoms. To be able to do that she would need to be released from the oath she once made to Lady Galadriel and she headed to Lothlorien in order to ask her to relieve her of her duty. Her response, however, as she’d soon find out, would be conditional.