Princess Basmah bint Saud bin Abdulaziz Al-Saud, a high-profile human rights advocate in the ultra-conservative kingdom, vanished after she attempted to leave Saudi Arabia for Switzerland in 2018.In a series of tweets on her official Twitter profile, Princess Basmah said she was abducted with her daughter without explanation and held in Al-Hair Prison - a maximum-security facility south of capital Riyadh - without criminal charges.Citing "very critical" health issues, the 56-year-old princess - who is the youngest daughter of Saudi Arabia's second ruler King Abdulaziz - asked the royal family for her freedom."My health is deteriorating to an extent that is serve [sic], and that could lead to my death," said Princess Basmah. "I have not received medical care or even response to the letters I dispatched from jail to the Royal Court."The official Twitter account of Princess Basmah's media office retweeted her pleas and shared a number of news articles pertaining to her detention.Hailing from a royal family that has faced fierce criticism over human rights abuses, Princess Basmah is known for her outspokenness on human rights.The princess has written a number of rights-related articles for various news outlets. She has also advocated for constitutional reforms and criticised uneven power structures in the kingdom.During a 2018 BBC interview, Princess Basmah called for an end to Saudi Arabia's military intervention in Yemen."I am still an obedient citizen and I will always be behind the royal family," she told The Independent in 2012."But I will never be quiet about what is happening on the ground."