Death Nine
She's a princess in this life, a step up from being the Capulet heiress. However, she's a girl and not even the firstborn- no, that title belongs to her brother Joffrey. So she gets to roam the castle to her heart's content unseen and unheard.
The only people that really pay attention to her are Septa Eglantine, Tommen, and Tyrion. To people other than them she is a ghost in the castle. The little joy she gets in this high society world comes with scaring members of the court as she pops out of hidden passages in the castle and listening to the servants gossip.
However, one night when she is nine, as her family 'enjoys' dinner, her uncle makes a remark about Dornish wine and women that makes Myrcella pause. The Dornishmen have been looked down upon in King's Landing for as long as she can remember, most of it stemming from the brutal murder of the former princess Elia Martell by one of the Lannister men. What if she changed that? What if she could ease the tension between the Lannisters, the Baratheon and the Martells?
It took some effort convincing Uncle Tyrion of her idea, but in the end they brought a plea to her father. They presented the idea of letting Myrcella go to Dorne with Tyrion to forge a more stable alliance with the Martells. Together with their combined intellect they managed to persuade him to approve the plan.
The preparations took almost a six months between the correspondence with Doran Martell to make sure everything was ready and assuring Cersei that Myrcella would be safe. She seemed convinced that the entire plan was Tyrion's fault and that he was going to marry Myrcella off the moment they got to Dorne. Surprisingly, it wasn't Jamie or even Myrcella that convinced her that Myrcella would be alright in Dorne, but Joffrey when in one of mother's fits he managed to soothe her with the fact that Myrcella would always come back from the viper's pit to the lion's den.
Myrcella had never been more grateful to finally leave King's Landing. Dorne was like a breath of fresh air to her, their clothes and food and people were much more open than those in King's Landing.
The next ten years she spends there as the war happens are happy and free. The people of Dorne love the foreign princess and she loves them. The best part of her decision to come to Dorne is Trystane. He is such a sweetheart and she can't help but fall in love with him.
It is that reason alone that gets her killed. The Lannisters have many enemies, and securing an alliance with Dorne through marriage would turn the tide in their favor.
Myrcella is nineteen when she is murdered in her sleep by a man wearing another's face.