Sef Ibn-La'Ahad (1197 – c. 1226) was a member of the Assassin Order, the youngest of Altaïr Ibn-La'Ahad and Maria Thorpe's sons, and Darim Ibn-La'Ahad's brother. He is also an ancestor of Desmond Miles.
In 1217, Altaïr, Maria and Darim left for Mongolia to assassinate Genghis Khan, though Sef elected to remain in Masyaf to take care of his wife and two daughters. Eight years later, the Assassin Abbas Sofian staged a coup d'état to take control of the Order for his own benefit. Abbas had his spy Swami, a feeble-minded Apprentice, stab Sef to death in his own bed. Swami told Sef just before carrying out the act that it was Altaïr who had ordered the execution.
Abbas later threw Malik Al-Sayf – the acting leader of the Order during Altaïr's absence – in the dungeons beneath the fortress, pinning the responsibility for Sef's death on the old Assassin. Two years after this, Altaïr, Maria, and Darim returned, having killed Genghis Khan, in order to learn what had transpired during their absence. Under the influence of false knowledge, Darim rode for Alamut to seek out his brother. Altaïr and Maria eventually discovered the truth, leading to the deaths of Malik and Maria at the hands of Swami and Altaïr destroying the former Apprentice with the Apple of Eden in his rage at losing his youngest son, ultimately finishing Swami off with his Hidden Blade.
After his death, Sef's family lived in Alamut along with his father and brother for a short period of time before they decided to move to Alexandria, driven away by Altaïr's depressed obsession over the Apple of Eden.
- "Sef" is an Egyptian name meaning "yesterday." The meaning of the name "Ibn-La'Ahad" is "son of none."
- Sef's children and their extended families were referenced in an internal e-mail sent between the higher-ups in Abstergo Entertainment. While discussing other potential candidates for exploration for the Sample 17 Project, the part of Desmond Miles' ancestry hailing from 13th-century Egypt was listed as a possibility.