According to the Rig Veda, Vritra kept the waters of the world captive until he was killed by Indra, who destroyed all the 99 fortresses of Vritra (although the fortresses are sometimes attributed to Sambara) before liberating the imprisoned rivers. The combat began soon after Indra was born, and he had drunk a large volume of Soma at Tvashtri's house to empower him before facing Vritra. Tvashtri fashioned the thunderbolt (Vajrayudha) for Indra, and Vishnu, when asked to do so by Indra, made space for the battle by taking the three great strides for which Vishnu became famous.