As described in a number of Purana, it was considered as a very destructive weapon. It is said that when the Brahmastra was discharged, there was neither a counterattack nor a defense that could stop it, except by another Brahmastra.
Karna didn't use Brahmastra against Ghatotkacha because he knew Arjun also had a Brahmastra to stop it. As Karna possessed a divine weapon Vasavi Shakti(-that Lord Indra had given to him), he saved that weapon for Arjun. But the Kaurava leader Duryodhana appealed to Karna to kill Ghatotkacha And he was unable to refuse Duryodhana, Karna discharged that weapon against Ghatotkacha.
The very first thing, as per Purans if two Bramhastra are used against each other. They will cause destruction of everything i.e. Earth n Nature.
As when Lord Ram used Bramhastra against Ravana, Ravan didn't used Bramhastra as he knw he had a boon from Brahma Dev, which will protect him from brahmastra. But if uses the Brahmastra against lord Ram Brahmastra then their wud b a complete annailation.
Likewise, a warrior like Arjun too knws, what can happen if two Bramhastra used against each other.
Also Bramhastra was the divine weapon of the Creator (Brahma), having power destroy everything thing created by him.
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