The word kiya means lord or ruler, as he was the second hujjat and ruler after Hasan bin Sabbah, who most probably was born in 455/1062 in the peasant family of Rudhbar. He passed most of his childhood in Rudhbar in cultivation of the land of his father. He was not, however, related by marriage to the local Caspian rulers, as is mentioned in few sources. In reality, it was a sister of a Zaidi ruler of Daylam, called Kiya Buzrug al-Da'i ila'l-Haq bin al-Hadi (d. 551/1156); and not Kiya Buzrug Ummid's sister, who is recorded as being the wife of Hazarasf bin Fakhr ad-Dawla Namavar, the Baduspanid ruler of Rustamdar and Ruyan. The latter's son Kayka'us (d. 560/1164), who adhered to Zaidism and ruled for 37 years, was the sworn enemy of the Ismailis. Hazarasf's grandson, Hazarasf bin Shahrnush (d. 586/1190), however, procured close relation with the Ismailis. It must also be remembered that a certain Kiya Buzrug, and not Kiya Buzrug Ummid, had married a daughter of Shah Ghazi Rustam bin Ala ad-Dawla Ali, who later became the Bawandid ruler of Mazandaran and Gilan between 534/1140 and 558/1163. The Bawandid was an Iranian dynasty who reigned from 45/665 to 750/1349. Shah Ghazi Rustam was an enemy of the Ismailis, and fought with them on numerous occasions with the help of the Seljuqids. His daughter was however married to the Baduspanid ruler, Shahrnush bin Hazarasf bin Namavar, who cemented cordial relations with the Ismailis.
Kiya Buzrug Ummid had been a handsome young page, whom Hasan bin Sabbah had converted before almost 480/1087. He played a leading role during the possession of Alamut in 483/1090. He was an outstanding organizer, talented dai and an able administrator. Hasan bin Sabbah had sent him with a troop to conquer the fortress of Lamasar in 489/1095. He thus defeated a certain Rasmasuj and took possession of Lamasar, also known as Rudhbar-i Alamut.
According to "Jamiut Tawarikh" (pp. 27-8), "The fort of Lamasar was situated on a rotten hill, with a few decayed houses on it, with no vegetation nearby. The climate of the place was very hot. Kiya Buzrug Ummid fortified the castle and cut the rocks to build a canal from a point on the Nine-rud, two and a half farsakhs away, which could supply water to the fort. The fort was thus irrigated. Water reservoirs were made and trees were planted and the fort began to look a royal rest house (khushk) in a garden. It was put in charge of Kiya Buzrug Ummid." Since then, he controlled the affairs of Lamasar till the death of Hasan bin Sabbah, and thereafter, he was summoned in Alamut to take charge of the Nizari Ismaili state in accordance with the orders of the Imam.
Kiya Buzrug was confronted with the animosity of the local amirs as soon as he assumed the power. In 518/1124, some 700 innocent Ismailis had been butchered mercilessly near the Postern gate (bab as-sirr) at Amid (the Roman Amida) in Diyar Bakr.
It appears that sultan Sanjar had refrained from launching further operations, possibly due to his pact with Alamut. When Alamut came to be governed by Kiya Buzrug, sultan Sanjar took militant stance to test the capabality of the new leadership of the Ismailis. In 520/1126, he sent a large army at the command of his vizir against Turaythith in Kohistan, as well as Bayhaq and Tarz in the district of Nishapur, with orders to massacre the local Ismailis and sack their properties. This expedition sent from Khorasan had been fissiled of no result. The expedition sent in the same year by sultan Muhammad to Rudhbar at the command of Asil, the nephew of Anushtagin Shirgar was repulsed by the Ismaili warriors. Another Seljuq attack in the same year was also foiled by the Ismailis of Rudhbar, who captured one of the enemy's amirs, Tamurtughan. He was taken prisoner to Alamut for some months. Tamurtughan was however released upon the request of sultan Sanjar.
During the rule of Kiya Buzrug, several other fortresses were seized, including Mansura and others in Taliqan, while a few castles were built, such as Sa'adatkuh and Mansura in 521/1127.
In 523/1129, the sultan Muhammad entered into peace negotiations with the Ismailis, and for this purpose, he invited Alamut to send an envoy to Ispahan. Kiya Buzrug dispatched Khoja Muhammad Nassihi Shahrastani. But the discussions proved abortive as the Ismaili emissary and his colleagues had been killed by some of the town people while they were leaving the Seljuq court. The sultan disclaimed the responsibility, also rejecting to punish the assassins. Thus, the Ismaili forces, in reprisal attacked Qazwin, killing 400 persons and taking away much booty.
Sultan Muhammad executed another unsuccessful raid at Alamut district, while an army was also sent from Iraq in 525/1131 against Lamasar with 30,000 soldiers, but of no avail. Meanwhile, Sultan Muhammad died and his army retreated, and after that the fortress of Lamasar was never invaded.
By the end of Kiya Buzrug's reign, the Ismailis had clearly established an independent state of their own. This state primarily consisted of two areas in Iran, namely Rudhbar and a large tract of Kohistan, as well as the southern part of the Jabal Bahra in Syria. At the same time, there were non-Ismailis, including Sunnis, Ithna Asharis, Zaidis and Nusairis living in the areas dominated by the Ismailis. The Nizari state had its own mint as an accepted territorial rule. Kiya Buzrug's adoption of the role of a territorial ruler and his acceptance by others as such, are strikingly demonstrated by the flight to Alamut in 530/1136 with his followers of a certain Seljuq amir Yaranqush, an old enemy of the Ismailis. He was dislodged from his iqta (an administrative grant of land) by Khwarazmshah and took refuge at Alamut. According to "Jamiut Tawarikh" (p. 142), the Shah asked for his surrender, arguing that he had been a friend of the Ismailis, while Yaranqush had been their enemy. Kiya Buzrug refused to deliver him to Khwarazmshah, saying: "I cannot reckon as an enemy anyone who places himself under my protection." This reflects indeed a chivalrous and greatness of Kiya Buzrug.
Kiya Buzrug Ummid excelled in his works that Hasan bin Sabbah had reposed in him. He died on 26th Jumada I, 532/February 9, 1138 after ruling for 14 years, and was buried next to the tomb of Hasan bin Sabbah. He was succeeded as the third ruler by his son Muhammad bin Kiya, whom he designated only three days before his death according to the order of the Imam. According to "Rawzatus-Safa" (4th vol., p. 78) that, "The enemies of Kiya Buzrug became joyful and insolent, but they were made soon to realize that their hopes were vain."