Brahmastra was a deadly arrow obtained after propitiating Lord Brahma. There was no defense against Brahmastra. The person who had discharged the arrow can call it back. It could only be stopped by Brahmadanda – a stick created by Brahma.
Brahmastra never misses it mark and it can only be used once in a day.
It is invoked by a mantra that is bestowed upon the user when Brahma blesses him with the arrow. The mantra is used to call upon the weapon when required.
The weapon can be used against an individual or against an entire army.
The arrow causes severe environmental damage. The land becomes barren and living beings cannot survive in such a land. There will be no rainfall. Those residing in the area become infertile.
Brahmastra was used in the Ramayana. Bhagavan Ram used it to annihilate Ravana.
Another famous usage of Brahmastra was when it was used by Arjuna and Aswathama against each other. Arjuna called his weapon back. But Aswathama directed it against the unborn grandchild of Arjuna.