In 1165, Umar's wife, Maud, died of complications from giving birth to their son Altaïr, leaving him to raise their child alone.
In 1176, the forces of Salāḥ ad-Dīn laid siege to Masyaf. After finding the location of Salah Al'din's tent that was revealed by the spy Ahmad Sofian, Umar was tasked with infiltrating Salah Al'din's tent and leaving a message there.
Successfully managing to infiltrate the tent and leave the message, Umar was just about to leave when he was discovered by Salah Al'din, forcing the Assassin to flee and kill a Saracen nobleman in the process.
When the Saracens moved to form a peace treaty, they demanded that Umar's life be taken in payment for the Saracen nobleman he had killed. However, if the Assassins refused, the Saracens would execute Ahmad and continue their siege of the city.
After a lot of deliberating and failed attempts to negotiate the terms, Rashid ad-Din Sinan, the Mentor of the Levantine Assassins at the time, had no choice but to allow Umar to take Ahmad's place. Moments before he was executed, Umar heard the voice of his 11-year-old son crying out for him from the top of the fortress.
The actions of Umar had many far-reaching consequences, even decades after his death. Ahmad, unable to live with his guilt and grief, committed suicide after apologising to Altaïr and Ahmad's son Abbas led the Brotherhood into decline and ruin as a result of refusing to face the truth. Altaïr would often reflect on his father later in life; when observing a book-burning being conducted by a Templar, Altaïr mused that the sight would have disgusted his father.
- Umar is an Arabic name meaning "prospering, thriving, flourishing", derived from عمر ('umr) meaning "life". The meaning of the name Ibn-La'Ahad is "son of none".