Indra battled the dragon Vritra, but Indra had sworn an oath to Vritra to not hurt it with weapons made of wood, stone, metal, or any other conceivable weapon, to not hurt it with any object dry or wet, and to neither attack it during the day or night. Indra attacked at twilight, neither day or night, and used a column of sea foam, not wood, stone, metal, dry, or wet, to defeat the dragon.
As the father of Arjuna, Indra favored his victory in what was to be the final fight between his son and Karna, and he understood that Karna could not be persuaded from battling. The armor Karna had been bestowed at birth, Kavacha and Kundala, was such that even a god would find it difficult to destroy, so he formed a plan to take it from Karna. Shortly before the battle, Indra took the form of a Brahmin due to Karna having sworn an oath to never refuse the request of ome. Approaching Karna while he was taking a holy bath, Indra requested "I wish to receive the things you carry." Although he knew of the trap, Karna offered up the requested item, the armor that was his only proof of his lineage.
Karna was not dissuaded from battle despite losing the armor, causing Indra to be touched by his gallant figure. He asked Karna why he proceeded to the battlefield despite that, without even reproaching he who had taken his armor out of his love for Arjuna. Karna replied that Indra was simply a step ahead, and although a god, his love as a father brings Karna joy. He explained his reasoning for proceeding to the battlefield, and Indra saw in Karna the "divinity of Surya himself." Despite being the enemy of his son, Karna's noble behavior charmed Indra to the point he came to adore him. Having taken something from Karna that was greater than his life, Indra decided to give Karna his spear, Vasavi Shakti, as compensation to avoid staining his honor. The greater reason that he was charmed by Karna and came to believe that Karna could wield the greatest spear he had not given even to his own son. After sending off Indra, Karna headed to the battlefield for his battle against Arjuna.
Indra possessed Vasavi Shakti, only permitting himself to use it. Not even passing it onto Arjuna, he later finds Karna worthy of possessing it. The Vajra is said to be the symbol of Indra's divinity and was held in one interpretation to be a manifestation of Indra’s lightning.