Brahmastra, capable of annihilating the whole world, is an arrow possessed by very few warriors in the ancient world. Arjuna possessed the arrow and he was blessed with it by Drona, his Guru in the Mahabharata.
Once when Drona and his students were bathing in the
a huge fish like species caught hold of the leg of Drona. The Guru tried
numerous means to pull his leg out but he could not move his leg from the mouth
of the huge fish. Ganga River
The students tried to pull out the Guru but they too failed. They tried numerous other means to save Drona but nothing yielded the desired result. The fish kept pulling Drona deep into the river.
On hearing the commotion, Arjuna appeared on the scene and immediately shot an arrow right on the head of the fish. The fish, writhing in pain, released the leg of Drona. The Guru then swam to safety.
Drona then rewarded his most favorite disciple for saving his life by imparting the knowledge of Brahmastra.
Arjuna was advised not to use Brahmastra against women and men who were not equal to him in warfare.