Karna received the Vasavi Shakti (Amogha) weapon from Indra in exchange for his kavacha (armor). He had sworn to use it only on Arjuna, but during the Kurukshetra war, Duryodhana pleaded with Karna to save them from Ghatotkacha's rampage. Karna, compelled to save the Kaurava army, used the divine weapon on Ghatotkacha instead, thereby sacrificing his one chance to kill Arjuna.