A follow up to this
Aidan was fond of Ceather ever since they met at Clonmacnoise, she was bright, charming, and compassionate to everyone she met. So by the time they crossed paths again at Kells, it was no surprise his feelings had grown for her. Ceather had grown into a beautiful woman and had overheard the news of Aidan's newly found fame as the master illuminator of Iona and was wanting to reconnect with each other. Before long, Cellach would catch his sister hanging around his best friend--- even more than her own brother. He was aware of Aidan's little crush on his sister, but never thought he would act upon it. He was a brother of Iona was a high reputation to uphold after all.
During Aidan's stay at Kells, Ceather would accompany him into the woods to help him find berries and quills for his work. Soon the excursions took longer and they'd stay passed sundown to stargaze.
However little did they know that they were being spied on by Brother Brutus who told Arsalan about their peculiar late night excursions. Arsalan then used this information to plot against Aidan to ruin his reputation at it's most highest just as he had ruined his and spread rumors throughout the brotherhood.
When Aidan had returned to join the brothers at the monastery, he grew suddenly uneasy by the countless stares and whispers. When he was unable to find Cellach, he asked Brother Rory if something was wrong, only to have Brother Rory ask him,
"Is it true you have broken your vows?"
"Is it true you have been sleeping with Cellach's sister, Ceather?"
Aidan's heart sank. He rushed to find Cellach to tell him the rumors were false, but he was then greeted by a firm punch to the face. Cellach was well aware of the rumors that had been disgracing his family and didn't believe Aidan's claims. Ceather then stepped in and tried to reason with her brother, but much to her horror, he only thought she was defending Aidan and disregarded her too.
Aidan then began wondering who could do this to them and immediately thought of the former Abbot, Arsalan. He confronted the blind monk in the round tower, but Arsalan knew there was nothing he could do to stop it. The former Abbot sneered with his crooked teeth as he sensed Aidan wanting to push him through the window of the tower to his death.
"I know you want to... Why stay as Brother of Sin, when you can become a Murderer instead?"
Defeated and powerless, Aidan reluctantly exited the round tower.
Though it was all a lie, Aidan couldn't help but blame himself every time he heard a man or woman gossip and scoff at the "Whore of Kells". It killed him inside whenever he found Ceather crying.
Heartbroken, Aidan tried to distance himself from Ceather to avoid adding more fuel to the fire, and began making pans to leave Kells. However, Ceather caught him that night and begged him not to go.
"Aidan... If you run away now, it only will confirm the rumors."
Aidan held her close, and promised he wouldn't leave her alone to suffer like this. He swore to find a way to help, but he wasn't sure how.
Most men in Ceather's village caught wind of the rumors-- about the whore of Kells-- though they had no idea who this monk was. However things changed when a stranger from the north came. Bronach. He too was avoided like an outcast due to him being a former Viking. However the one person who dared come to him was Ceather and before they both knew it, Bronach fell in love with her. He even saved hurt from a pack of wolves when searching for food.
Aidan took notice and began watching over Ceather in case things turned ugly. But he could see Ceather was starting to have feelings for him too and decided to get to know him. Before long they became friends and Bronach educated him on his Viking background and folklore. When Aidan felt he could trust him enough, he shared what happened between him and Ceather at the abbey. Bronach was understanding and promised to love and watch over her as she deserved.
In secret, Aidan had them married with the help of Ceather's mother, Torra, who was one of the few people to listen to reason. Aidan knew this would damage Cellach's trust in him even more, but he knew he had to help Ceather in her happiness.
Later, Arsalan had caught rabies from a rat and plugged down the stairs of the round tower in his madness, breaking his skull and neck. It wasn't too long before Brutus hung himself from the cross of Kells, having no other purpose to continue on living when he's done so much wrong. At his feet he left a paper of his final confessions to the murder of Matthias and proclaiming the innocence of Aidan and Ceather. But by that time, it was too late for Cellach to truly make amends after discovering his sister was now married to a viking and Aidan had been behind it. Although he managed to forgive Ceather, he never saw eye to eye with Bronach and deep down still resented Aidan for causing such disgrace to his family. He still had a spark of jealousy that Ceather put her trust on Aidan instead of her own brother.
It only got worse years later when Brendan and Aidan suddenly got closer. Like mother like son.
The Secret of Kells (C) Cartoon Saloon & Tomm Moore