Wikipedia
Abassa (also known as Abbasa) (born 765 – died after 803) was an Arabian noblewoman.
She was the daughter of Al-Mahdi. Her mother was a concubine by the name of Rahim. She was the half-sister of Al-Hadi, Harun al-Rashid, Ulayya, and Ibrahim ibn al-Mahdi. Harun al-Rashid was known for being unhappy with the fact that he was a relative of Abassa's, as he was attracted to her. To keep Abassa in his life, he had her marry Ja'far ibn Yahya. The marriage was supposed to be one of convenience, but, Abassa fell in love with her arranged husband. At night, a slave woman would be sent to Ja'far's bedroom, and Abassa took the slave woman's place one evening. Her husband was surprised. Abassa would get pregnant and give birth to twin boys in secret. The twins would be raised in Mecca. Harun eventually found out about the relationship, and had Ja'far killed. Abassa was either killed, or, sent into exile.