#casella vaith

LADY CASSELLA VAITH Daughter of a Dornish lord After the Submission of Sunspear, Aegon escorted the hostages that the king had gathered from the lords of Dorne back to King's Landing. Among them was Cassella Vaith, a willowy maid with green eyes and pale white-blond hair, whom Aegon ended up keeping "hostage" in his own chambers. When the Dornishmen revolted and murdered King Daeron, all the hostages were to be killed, and Aegon—by then bored of her—returned Cassella to her place with the other prisoners. However, the new king, Baelor, pardoned all the hostages and personally took them back to Dorne. Cassella never wed, and in her old age she was consumed by the delusion that she had been Aegon's one true love and that he would soon send for her. Children by Cassella Vaith: None. (The World of Ice and Fire - The Targaryen Kings: Aegon IV)

She was taken hostage in 158 AC during king Daeron I's Conquest of Dorne, old enough to be considered a maid, and was treated as spoils of war and sexually assaulted by a then 23-year-old Prince Aegon (later crowned The Unworthy). In 161 AC, she was turned over to the hostages awaiting execution for the death of king Daeron, but ultimately pardoned and returned home by the new king Baelor. That's around 3 years in captivity as Aegon's "mistress" aka. victim.

Next we hear of her, she is an anecdote of Dunk and Egg's time in Dorne. Dunk was rude to her, likely in part because he was still grieving the loss of his horse Chestnut.

The stot had died on the long thirsty crossing between the Prince's Pass and Vaith, with Egg upon his back. [...] Chestnut had died on the long dry ride to Vaith, that part was true. [...]  "I don't know how to talk with highborn ladies," he confessed as they were pouring. "We both might have been killed in Dorne, on account of what I said to Lady Vaith." "Lady Vaith was mad," Egg reminded him, "but you could have been more gallant. Ladies like it when you're gallant. [...] Dunk had doubts. "She's outlived four husbands, she must be as old as Lady Vaith.  (The Sworn Sword)

And another mention in The Mystery Knight, having impressed Dunk in some unspecified way enough to be worth remembering in this context.

Voices echoed in his head, and faces drifted past him—old Ser Arlan, Tanselle Too-Tall, Bennis of the Brown Shield, the Red Widow, Baelor Breakspear, Aerion the Bright Prince, mad sad Lady Vaith. 

He met her in 209/2010 AC, so this was 52 years after she was abducted from Dorne as a girl, and she is still around and kicking, even as she is old and "mad". We don't know how old she was when she was kidnapped, but likely a teenager rather than an established adult woman. Perhaps the experience was so traumatic that she had to reframe reality in order to cope with it, a mechanism we have seen employed in similar situations, with Daenerys and Drogo, Sansa and Joffrey, Jon and Ygritte. In a different take, she might be more similar to Falia, who is equally delusional about her true status with the man who captured her family, though with a more malicious bend. Then there's Lysa with Littlefinger, though the power dynamics are reversed.

Either way, it's not surprising that she never married and stayed locked inside her traumatic youth for the rest of her life.