Srimad Bhagavatam Canto 08, Chapter 11, Text 35

SB 8.11.35

tapah-saramayam tvastram
 vrtro yena vipatitah
anye capi balopetah
 sarvastrair aksata-tvacah
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Vrtrasura was the essence of the austerities undergone by Tvasta, yet the thunderbolt killed him. Indeed, not only he but also many other stalwart heroes, whose very skin could not be injured even by all kinds of weapons, were killed by the same thunderbolt.
Srimad Bhagavatam Canto 08, Chapter 11, Text 34
Srimad Bhagavatam Canto 08, Chapter 11, Text 36