The earliest known king of Magadha is Brihadrath. Name of Brihadrath appears in Rig-Veda (I.36.18, X.49.6). Magadha is mentioned in Atharvaveda. The puranic sources say that Brihadrath was the eldest son of Vasu. According to Ramayana, Vasu founded Vasumati and Grivraja. The Brihadrath dynasty was founded by Brihadrath. His son was Jarasandha.
Name of Jarasandha appears in Mahabharta and Puranas many times. Jarasandha was inimical to Yadavas and that is why he is mentioned as a villain in the Mahabharta. Jarasandha was a powerful king and a devotee of Shiva who at the time of his birth was in two pieces of human body born to two wives of Brihadrath. These two pieces were thrown into the forest on the order of Brihadrath. In the Forest, a Rakshashi known as Jara found these two pieces. When she joined them, they became one and thus the name Jarasandha (one who was joined by Jara was given).
In the Mahabharta war, Jarasandha was killed by Bhima. The time period assigned to Jarasandha is approximately 1760 BC. He was succeeded by Sahadev who also got killed in the epic war of Mahabharta.
The Brihadrath Dynasty ended approximately 8th century BC.