Krishna Explains His Absence to Yudhishthira
Yudhishthira asks Krishna why he was absent after the gambling match. Krishna reveals that while Yudhishthira was losing everything in Hastinapura, he was fighting a war of his own — against Shalva, who had attacked Dvaraka in revenge for Shishupala's death, riding a flying city called Soubha.
Yudhishthira asked Krishna directly: Why were you absent? Where did you go? What did you do on your travels?
Krishna answered.
He had gone to destroy Soubha — the flying city of Shalva. And the reasons began with Shishupala.
At Yudhishthira's rajasuya sacrifice, Shishupala had been killed by Krishna — struck down in the assembly over the question of who should receive the first homage. Shishupala was Shalva's brother. When Shalva heard the news, he was filled with terrible rage. And he acted on it while Krishna was still with Yudhishthira.
Shalva ascended Soubha — a city that could go wherever it wished — and flew to Dvaraka. The city was empty of Krishna. But it was not empty of Vrishnis. Shalva fought the young bulls of the Vrishni lineage in cruel fashion. He slew many of them. He destroyed every park in the city. And then he shouted, demanding to know where Krishna was.
"Where is that wretch of the Vrishni lineage? Where has the evil-souled Vasudeva gone? I will destroy his pride in battle. Tell me truthfully where he is. I will return only after killing the slayer of Kamsa and Keshi. I swear on my weapons — I will not return without killing him."
He rushed from one place to another, the lord of Soubha, searching for battle. He called Krishna treacherous, mean, evil. He said Shishupala had been a child, killed when he wasn't ready, not in proper battle. He swore he would kill Janardana.
Krishna heard all of this when he returned to Dvaraka — the abuse, the oppression of Anarta, the insolence, the evil deeds. His eyes turned red with rage. He made up his mind. He resolved to kill Shalva.
He left to destroy Soubha. He found Shalva whirling in the ocean. He blew on Panchajanya — his conch, obtained from the water — and challenged Shalva to battle. A terrible battle followed between Krishna and the danavas (demonic beings) there. They were all vanquished, laid low on the ground.
That was what kept him from coming to Hastinapura, though he had heard about the terrible gambling match. Aranyaka Parva, Chapter 312