Speaking in London after having escaped her captors, she appealed for help for those who remained trapped and for her Yazidi community, a minority which IS has targeted for massacres, enslavement and rape in northern Iraq.
"It's not a life. We are not living until the rest of our people are saved from Daesh," Nihad Barakat al-Awsi told AFP, using an Arabic acronym for IS.
The teenager was kidnapped with 28 members of her family in Sinjar in northern Iraq in August 2014 during a persecution campaign by IS militants, who consider the Yazidis heretical.
Two of her six sisters and two of her 12 brothers are still in the hands of the jihadists, she said in an emotional hour-long account of her trauma, nervously clasping her knees and shedding discrete tears.
Al-Awsi was speaking on the sidelines of a conference by the British charity Amar on the "urgent need" to address mental health issues amongst refugees and internally displaced people in the Middle East.
Baroness Emma Nicholson, the charity's chairman, spoke of the "great, great need of psycho-social help as well as physical help" for the displaced.
Thousands of Yazidis like al-Awsi were killed or kidnapped by IS jihadists and many are still missing.
She said some members of her family managed to escape when their two cars were stopped at a checkpoint as they tried to flee to the mountains for safety.
|They chose 21 girls. They put them in a room and the rapes started
- Nihad al-Awsi
The militants took her and around 300 Yazidi families to a school in Hasakah in Syria, a town close to the Iraqi border.
"During the night, they separated the men from the rest of the families," she said, speaking in Arabic through an interpreter. "They came and said: 'Either you become Muslims or we will assassinate the men'," she said.
After being taken back to Iraq "they separated the girls" and took them to a gym in Mosul to "marry" them. But they refused to submit and were beaten for two weeks.
Then a local emir - or leader - and his men arrived.
"They chose 21 girls. They put them in a room and the rapes started," she said.
Her rapist's child
She was "chosen" by Salam, a 25-year-old militant.
After raping her, he took her to his family home where he lived with his pregnant wife and their son, and the sexual violence continued.
"His wife wasn't very nice. She said I invaded her and her home," she said.
A month and a half later, Salam was killed in Syria.
Al-Awsi managed to escape but was captured again in Kirkuk and taken to IS-held Mosul. She was handed over to an emir's brother - a father of four daughters who also raped her.
|I wanted him to have a Yazidi name but his father named him Issa. It means Jesus.
- Nihad al-Awsi
He told her if she became Muslim he would take her back to her family. She accepted and later found out she was pregnant, avoiding more rapes, and tried in vain to terminate her pregnancy.
She refused her abusers' proposal of marriage despite his insistence, and gave birth to a baby boy.
"I wanted him to have a Yazidi name but his father named him Issa. It means Jesus."
Three months after the birth she managed to call one of her brothers through her neighbour to arrange an escape.
"Without Issa" she said, tearfully. "I couldn't bring him to my family."
On 15 October, 2015 a smuggler took her into Iraqi Kurdistan where she was reunited with members of her family in a refugee camp.
Despite the haunting memories, she is looking to the future: "I would like to finish an education. I want to study English and I want to get married and have a family."