Magadha Empire was ruled by Sisunga Dynasty during sixth century B.C.
Four important empires during Sixth century B.C were
1) Haranyakas of Magadha
2) Ikshvakus of Kosala
3) Pauravas of Vatsa
4) Pradyotas of Avanti
The Notable emperors of Magadha empire were Bimbisara and his son Ajatsathru. Bimbisara founded the Magadhan Empire overthrowing Brihadrathas. Bimbisara anointed his son as king at the age of 15. The notable achievement of Bimbisara was annexation of Anga. He also entered in Matrimonial alliances with Vaishali and Koshala which made him expand his kingdom to the borders of Nepal.
Gautama Buddha and Vardhaman Mahavira preached their doctrines during the reign of Bimbisara.He was friendly to both Buddhism and Jainism and he suspended tolls at the river Ganges because Buddha was once stopped at the Ganges river due to lack of money. Bimbisara built a modern town called Rajgir or Rajgriha.
Tradition affirms that he was killed by his son Ajatsatru, later his wife also died in grief. This led Prasanajit, the king of Kosala to revoke the gift of Kashi, which triggered war between Magadha and Koshala. Ajatsatru was captured in an ambush and later his army and himself were restored in Magadha under the treaty of peace. Later he married Prasanajit's daughter. Inorder to repel the attacks of Vaishalis he built a fort in the village of Pataligrama which stood in the confluence of Ganga and Sone rivers. After generations it became the stately city of pataliputra.
At the beginning of the fifth century Pauravas and Pradyotas began to retire from the contest for supremacy, which thus left Harayankas of Magadha and Ikshvakus of Kosala. After continuous battles Magadha emerged as the supreme power in North India, which lead Ajatsatru to be founder of Magadhan supremacy. Udayi was the immediate successor of Ajatsatru, who as per the Buddhist scripts was unworthy to rule.
Later Magadhan Empire was taken over by Sisunga dynasty since it was under the lineage of Sisunga dynasty.