Saif al-Dīn G̲h̲āzī I bin ʿImād bin Ḳasīm al-dawla Aḳsonḳor bin ʿAbd Allāh, atābeg al-Mawṣil

He was the eldest son of Zengi of Mosul, and the elder brother of Nur al-Din.

In 1146 Zengi was besieging the fortress of Qal Ja'bari when he was assassinated on September 15 by one of his servants who wanted to escape punishment. His forces were scattered, but Zengi's two sons were able to regain control and to divide informally the empire: Saif ad-Din succeeded him in Mosul and the Jezirah (northern Iraq) while Nur al-Din succeeded in Aleppo. Saif al-Din had first to fight to secure his position in Mosul.

Two years before, the Seljuk sultan Ghiyath ad-Din Mas'ud had named his cadet son Alp-Arslan as overlord of Zengi, but the latter had neutralized him and carried with him at the siege. At Zengi's death, Alp-Arlsan tried to exploit the ensuing disorder to gain the power in Mosul. Two of Zengi's advisors, the head of the diwan al-Din Muhammad Jemal and hajab Amir Salah al-Din Muhammad al-Yaghisiyani took the side of Saif ad-Din: taking advantage of the inexperience of the young Seljuk, giving Saif al-Din the time necessary to take control of Mosul. When Alp Arslan appeared in Mosul, he was arrested and imprisoned in the citadel.

In 1148, together with Nur al-Din, he marched south to help defend Damascus during the Second Crusade (see Siege of Damascus). The atabeg of the city, Mu'in ad-Din Unur, however refused them entrance, using the presence of Zangi's sons to convince the Franks to release the siege. He died in November 1149 and was succeeded by another brother, Qutb ad-Din Mawdud.

https://en.wikipedia.org/wiki/Saif_ad-Din_Ghazi_I