Question
What is the story behind the Brahmasthra? Was it actually ever fired?
Answer
The Brahmastra is the weapon of Brahma. Like other astras (celestial weapons), it is invoked with a special mantra, and then you can use it to destroy anything that is created by Brahma. The Brahmastra does refer to a special arrow, but an ordinary arrow can also be given the same power using the mantra, and as discussed below, some people have even used the mantra on blades of grass when they didn't have arrows handy.
There are numerous occasions in which the Brahmastra has been used:
As described in the Sundara Kanda of the Ramayana, for some reason Indra's son Jayanta once turned into a crow and started pecking at Sita when Rama and Sita were on the Chitrakuta mountain. The pecking soon grew violent enough that Sita started bleeding, and Rama loved Sita so much that even her smallest injury was intolerable to him. So he turned a blade of Kusha grass into a Brahmastra and threw it at the crow, but then redirected it to merely destroy the crow's right eye when it begged for mercy:
Then, the long-armed Rama, the best among wise men, swirling his eyes in anger, made a resolve in the matter of that ferocious crow. Taking a blade of Kusa grass from his bed ( made of Kusa grass), Rama employed it to work with Brahma's missile (a mythical weapon which deals with infallible destruction). That blazing shoot of grass, resembling a fire destroying the world, flared up in front of that bird. As Rama threw that blazing blade of Kusa grass towards that crow, that blade of grass went chasing that crow in the sky.
Then, while that blade of grass came chasing, that crow went flying in many a way. Seeking protection, it roamed all over the world. Roaming the three worlds in search of a saviour, that crow was abandoned by Indra; its father, the celestials and the sages. Finally, it sought refuge in the same Rama.That Rama, who affords protection, was compassionate and protected that crow, which fell on the ground (in salutation to Rama) and sought for protection, eventhough it was apt to be killed. Seeing that crow, coming exhausted and dejected, Rama said to it: "It is not possible to make Brahma missile a waste. For this reason, tell me what to do now." Thereafter, that crow said "Let your arrow shoot my right eye." Then that blade of Kusa grass shooted the right eye of that crow. By giving away its right eye in that way, the crow saved its life.
By the way, this is the story that Sita told Hanuman, to give Rama proof that Hanuman really talked to Sita.
As described later on in the Sundara Kanda of the Ramayana, after Hanuman talked to Sita in Lanka, Ravana's forces tried to attack him, but they were unsuccessful. So then Ravana's son Indrajit launched a Brahmastra at Hanuman. Now as I discuss in this answer, Hanuman had received a boon from Brahma that he couldn't be destroyed by any weapon, so the Brahmastra only had the effect of immobilizing him and knocking him to the ground. And due to further boons from Brahma even that effect wore off quickly, although Hanuman continued to lie motionless just so that he could be captured.
As described in this chapter and this chapter of the Yuddha Kanda of the Ramayana, when Rama needed to cross the ocean to go to Lanka, he prayed for three days straight to Varuna the ocean god. But Varuna didn't appear before Rama, so Rama got angry and threatened to fire a Brahmastra to destroy the ocean (so that he could at least walk across the ocean floor to Lanka):
While Rama was stretching that bow, both the heaven and earth suddenly seemed to be split asunder. Mountains also were shaken. Darkness enveloped the world. All the quarters were obscured. Lakes and rivers were soon agitated. The moon sun and the stars moved obliquely and though the sun's rays lighted the sky, it was enveloped by darkness and shined with a blaze of hundreds of meteors while thunders reverberated with an unparalleled sound in the sky. Series of celestial winds blew in their colossal form and the winds, then sweeping away the clouds, tore up the trees again and again. The wind, shattering the mountain peaks, broke off the points of the rocks. Winds of great velocity struck together in the sky and emitted flashes of radiance proceeding from lightning with a great sound and then there were great thunders. The visible creatures cried out along with the thunders. The invisible beings too gave off a terrific noise.
The creatures were overpowered, frightened, agitated, lied down and also very much anguished. They did not move due to fear. The great ocean with its waves and water, along with its living creatures including snakes and demons soon became possessed of a terrific velocity. Due to that speed and swelling of waters, the ocean crossed beyond the other shore to the extent of a Yojana (eight miles). Rama born in Raghu dynasty and the annihilator of enemies, did not retreat before that ocean, which swelled and crossed its limits. Then, Sagara (the ocean god) himself rose from the middle of the ocean as the sun rises at dawn from the huge mountain of Meru.
So Varuna appeared before Rama and told him to build a bridge out of rocks that would float magically by the power of Nila and Nala. Rama was satisfied by this, but he still had to release the Brahmastra at something, so at Varuna's suggestion, instead of destroying the ocean water he destroyed the water at a place called Drumatilya frequented by robbers. That place, modern-day Malwar in Rajasthan, is now a desert as the Brahmastra destroyed the water there.
As described in a later chapter of the Yuddha Kanda of the Ramayana, in Rama's final battle with Ravana, Rama kept cutting off Ravana's heads but they kept regrowing. So then at the suggestion of Indra's charioteer Matali, Rama fired a Brahmastra (an actual Brahmastra arrow given by Agastya) at Ravana, which is what killed him:
Then, the valiant Rama, who was reminded thus by Matali, took hold of a blazing arrow, which was given by Brahma and which in turn was given to him by the glorious sage, Agastya earlier in the battle-field and which looked like a hissing serpent. Having been made formerly for Indra, the lord of celestials by Brahma, the lord of creation of infinite strength, it was bestowed in the past on the ruler of gods, who was desirous of conquering the three worlds...
Making it sacred by a special formula as per the procedure specified in scriptures, Rama who was endowed with an extraordinary strength then fixed that arrow which was the foremost among the three worlds, capable of removing the fear of Ikshwaku dynasty, taking away the glory of the enemies and bestowing joy to one's own self on his bow.
While that excellent arrow was being fixed by Rama, all the beings were frightened and the earth trembled. That enraged Rama, stretching his bow well and with an attentive mind, hurled that arrow which can tear off the vitals, towards Ravana. That arrow, which was inviolable as a thunderbolt hurled by the arms of Indra and irresistible as Yama the lord of Death, fell upon Ravana's chest. That arrow, released with great speed and which was capable of destroying the body, tore off the heart of that evil-minded Ravana. That arrow, which was capable of causing death to the body, after taking away the life of Ravana and having been anointed with blood, penetrated the earth.
As I discuss in this answer, the Sauptika Parva of the Mahabharata describes how after the Mahabharata war, Drona's son Ashwatthama tried to take revenge on the Pandavas by burning them in their tent while they were sleeping. But the Pandavas weren't at their camp that night, so Ashwatthama accidentally killed the Upapandavas, the five sons the Pandavas had with Draupadi. When he realized that he had killed five innocent boys, he went to the Ashram (hermitage) of Vyasa to repent for his sin. Then Krishna the Pandavas found out that Ashwatthama had killed their sons, so they tracked his chariot to Vyasa's hermitage.
When he saw them, Ashwatthama feared for his life, so he turned a blade of grass into a Brahmashirastra and launched it at them. (A Brahmashirastra, or "heads of Brahma" weapon, is like four Brahmastras in one!) And in response Arjuna launched a Brahmashirastra right back at him. To prevent the two weapons from doing extreme damage by colliding, the sages Narada and Vyasa came and persuaded Arjuna to withdraw his Brahmashirastra. Vyasa asked Ashwatthama to do the same, but he couldn't withdraw his weapon, so instead he redirected it to hit the wombs of the Pandava women. This did no serious damage, because although it killed the unborn baby Parikshit in the womb of Abhimanyu's wife Uttara, Krishna was able to bring it back to life.
Note: “The question: What is the story behind the Brahmasthra? Was it actually ever fired?” is licensed by Stack Exchange Inc (https://hinduism.stackexchange.com/); user contributions licensed under CC BY-SA.