Complex sentence: When Indra threw Vritra down, he crushed the fortresses that had already been shattered, and for this feat, Indra became known as Vritrahan "slayer of Vritra" and also as "slayer of the first-born of dragons".
Split into simpler sentences: When Indra threw Vritra down, he crushed the fortresses that had already been shattered. For his slaying of the dragon, Indra became known as Vritrahan "slayer of Vritra" and also as "slayer of the first-born of dragons".