The father of the regicide and also nephew of King Al Faisal was the twelfth son to who?
Musa'id bin Abdulaziz Al Saud:  He was the twelfth son of King Abdulaziz, the founder of Saudi Arabia. Faisal bin Musaid: Faisal bin Musaid bin Abdulaziz Al Saud (4 April 194418 June 1975) (Arabic: الأمير فيصل بن مساعد بن عبد العزيز آل سعود‎ ‎ , "ِ Fayṣal bin Musāʿid bin ʿAbd al-ʿAzīz ʾĀl Saʿūd ") was the regicide and also nephew of King Al Faisal.
King Abdulaziz