Vajra (king)
According to the Mahabharata and the Puranas, Vajra, (in Sanskrit: वज्र or वज्रा), also known as "Vajranabh" (वज्रनाभ्), was one of the last survivors of the Yadu dynasty. He succeeded his father King Aniruddha to the throne of Mathura. He was the great grandson of Shri Krishna and the grandson of Pradyumna.
Shri Krishna's wife, Rukmini, gave birth to the great warrior Pradyumna, one of Krishna's prominent sons, who later married the daughter of his maternal uncle Rukmi, Rukmavati. The mighty Aniruddha, was born to them, who married Usha, the Daitya princess. The princess gave birth to Vajra, who was known as the invincible warrior and would remain among the few survivors of the Yadus' battle. The name Vajra means "thunderbolt" or "diamond". A Vajra is a virtual object that represents firmness of spirit and spiritual power.
Story according to the Mahabharata and the Puranas
Crowning of Vajra
After Krishna left for Vaikunta, about 36 years after the Kurukshetra War (3138 BC), most of the major Yadu leaders had long died either due to disputes among themselves or the submergence of Dwarka into the sea. Arjuna went to Dwarka to bring the last surviving Yadus (i.e. Krishna's great grandson Vajra and the Yadava wives and elderly people) to safety in Hastinapur. Arjuna then proclaimed Vajra as the king of Mathura at Indraprastha. King Vajra's lineage is traced to the royal family of Jaisalmer.