Saubha was the name of the kingdom of Śālva, and also the name of his great flying city-fortress, a vimāna (flying city or car) made by divine craft. The two meanings ran together: Śālva was called “the lord of Saubha”, and the vimāna was called “the Saubha”.

How it was made

One account says the Saubha was built by Mayāsura. Another account says it was granted by Bhagavān Śiva.

Its powers

The Saubha was one of the most fearsome weapons of the Dvāpara Yuga. It could fly through the sky at will. It could also vanish and appear again, going from sight to unseen and back, so that forces on the ground or with ordinary weapons found it very hard to fight. It could rain destruction from above, with boulders, fire and other mighty forces, over a wide area. Because it moved through the air, it could strike from directions and heights that no ordinary defence was ready for.

The attack on Dvārakā

Śālva used the Saubha to attack Dvārakā after the death of Śiśupāla. The siege brought great ruin on the city before Bhagavān Kṛshṇa came back. Bhagavān Kṛshṇa then struck the Saubha with his divine weapons and shattered it. Its pieces fell into the ocean.

The full account is the Saubha Vadha episode of the Mahābhārata.

Sources

Mahābhārata, Vana Parva, Chapters 14–16