In 1173, after the death of his father, Ardashir I ascended the Bavandid throne. Right after the accession of Ardashir, his kingdom was invaded by the Khwarazmian prince Sultan Shah and the ruler of Khorasan, Mu'ayyid al-Din Ai-Aba, who captured several fortresses and cities from Ardashir. One year later, however, Mu'ayyid al-Din Ai-Aba was killed by Sultan Shah's brother Tekish. Ardashir quickly used the opportunity to reconquer Damghan and Bastam. Ardashir shortly made an alliance with Tekish, and made an agreement that when Tekish's daughter became old enough, she should marry Ardashir. In 1183, Tekish's daughter was sent along with her mother to the Bavandid capital of Sari, at which time the Oghuz leader Malik Dinar plundered the eastern parts of Mazandaran. Tekish shortly arrived at Gorgan, and forced the Oghuz leader to leave Tabaristan. He then demanded that Ardashir cede Gorgan to him, to which Ardashir was forced to agree. Ardashir also shortly married Tekish's daughter.

Ardashir now focused on his Paduspanid vassals, who were getting too powerful; he took much of Kai Ka'us I's territory which had been given to the latter during the reign of Ardashir's grandfather Shah Ghazi Rustam IV. Ardashir also ordered his general Mubariz al-Din Arjasf to attack Kai Ka'us. Sometime later, however, Kai Ka'us' son Justan died, and only his 1 year old grandson Zarrinkamar survived, whom Ardashir I agreed to raise and make ruler of the Paduspanid dynasty when the latter had grown up. Kai Ka'us shortly died in 1184. After his death, the nobles of the Paduspanid capital of Ruyan made his nephew Hazarasp II the new ruler of the dynasty. Unlike his predecessor, Hazarasp was a tyrant; he lost allegiance from several of his vassals by having some of their relatives killed, and making peace with the Ismailis, who were the most hated enemy of the Bavandids and several local rulers.

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