Ghiyath ad-Din Masud was the son of sultan
Muhammad I Tapar and his wife Nistandar Jahan also known as Sarjahan Khatun. At the age of twelve (1120–1121), he rebelled unsuccessfully against his elder brother,
Mahmud II, who however forgave him. At Mahmud's death in 1131, the power was contended between Mahmud's son, Dawud, Masud, whose powerbase was in Iraq, Seljuq-Shah (in Fars and
Khuzistan) and
Toghrul II. In 1133 Masud was able to obtain recognition as sultan from the emirs of
Baghdad, and to receive the investiture by caliph
al-Mustarshid. Toghrul, who controlling the eastern provinces of the western Seljuq, launched a military campaign but was defeated by Masud in May 1133. Toghrul died in 1134. Also in 1133 Mas'ud supported
Zengi, besieged by al-Mustarshid's troops in
Mosul. In 1135 caliph al-Mustarshid contested his authority but, on 14 June of that year, he was defeated and made prisoner at Daimarg, between
Hamadan and Baghdad, and killed two months later by the
Hashshashins. As al-Mustarshid's successor,
al-Rashid, also rebelled with the support of Zengi, Mas'ud
besieged him in Baghdad, forcing him to flee to Mosul, where he was also killed by the Hashshashins. Although able to ensure control over Iraq, Mas'ud's power over the rest of the Suljuq empire was uncertain: apart from
Khorasan and
Transoxiana, which had been long time under the control of his uncle
Ahmed Sanjar, Dawud kept control over Azerbaijan for several years, while weastern Persia was effectively ruled by emir Bozaba until Mas'ud defeated him, together with other emirs, in 1147. In 1148 Mas'ud faced another coalition against him, this time aiming to place
Malik Shah on the throne in his place. During his troublesome reign, Masud was forced to accept to delegate his authority to numerous emirs with the ''
iqta''', a tax institution which reduced the imperial incomes. Other became effectively independent sultans, such as
Zengi. This caused, according to historian
ibn al-Athir, the beginning of the steep decline of the Seljuq Empire. ==Death and legacy==