The valiant Aswatthaman, then, staying resolutely on his car, touched water and invoked the Agneya weapon incapable of being resisted by the very gods. Aiming at all his visible and invisible foes, the preceptor's son, that slayer of hostile heroes, inspired with mantras a blazing shaft possessed of the effulgence of a smokeless fire, and let it off on all sides, filled with rage.
The heat generated by the weapon slowly started to scorch the entire universe and it felt like universal dissolution which Samvarta fire carried out in the end of a yuga.
Endued with fiery flames, those arrows encompassed Partha on all sides. Meteors flashed down from the firmament. A thick gloom suddenly shrouded the (Pandava) host. All the points of the compass also were enveloped by that darkness. Rakshasas and Pisachas, crowding together, uttered fierce cries.
Inauspicious winds began to blow. The sun himself no longer gave any heat. Ravens fiercely croaked on all sides. Clouds roared in the welkin, showering blood. Birds and beasts and kine, and Munis of high vows and souls under complete control, became exceedingly uneasy. The very elements seemed to be perturbed.
The sun seemed to turn. The universe, scorched with heat, seemed to be in a fever. The elephants and other creatures of the land, scorched by the energy of that weapon, ran in fright, breathing heavily and desirous of protection against that terrible force. The very waters heated, the creatures residing in that element, O Bharata, became exceedingly uneasy and seemed to burn.
From all the points of the compass, cardinal and subsidiary, from the firmament and the very earth, showers of sharp and fierce arrows fell and issued with the impetuosity of Garuda or the wind.
Struck and burnt by those shafts of Aswatthaman that were all endued with the impetuosity of the thunder, the hostile warriors fell down like trees burnt down by a raging fire. Huge elephants, burnt by that weapon, fell down on the earth all around, uttering fierce cries loud as the rumblings of the clouds. Other huge elephants, scorched by that fire, ran hither and thither, and roared aloud in fear, as if in the midst of a forest conflagration.
The steeds, O king, and the cars also, burnt by the energy of that weapon, looked, O sire, like the tops of trees burnt in a forest-fire. Thousands of cars fell down on all sides. Indeed, O Bharata, it seemed that the divine lord Agni burnt the (Pandava) host in that battle, like the Samvarta fire consuming everything at the end of the Yuga.
The weapon burnt 1 akshouhini of Pandava army and soon spread it destruction. However, Arjuna APPARENTLY countered it with Brahmastra..
So it should be less powerful than Brahmastra? Well, that's the shocker.. Ashwathama' was shocked to see such an incident, he fled the scene and encountered Vyas and asks him how Krishna and Arjuna was able to escape with their lives. The weapon he generated was impossible to be countered and should've killed both Krishna and Arjuna. He asked Vyasa what was the reason for his Weapon’s failure.
'O sire, O sire, is this an illusion, or is it a caprice (on the part of the weapon)? I do not know what it is. Why, indeed, hath my weapon become fruitless? What breach (has there been in the method of invocation)? Or, is it something abnormal, or, is it a victory over Nature (achieved by the two Krishnas) since they are yet alive?
It seems that Time is irresistible. Neither Asuras, nor Gandharvas, nor Pisachas, nor Rakshasas, nor Uragas, Yakshas, and birds, nor human beings, can venture to baffle this weapon shot by me. This fiery weapon, however, having slain only one Akshauhini of troops, hath been pacified.
This exceedingly fierce weapon shot by me is capable of slaying all creatures. For what reason then could it not slay Kesava and Arjuna, both of whom are endued with the attributes of humanity? Asked by me, O holy one, answer me truly. O great Muni, I desire to hear all this in detail.'
Vyasa narrated the story of Krishna’s previous birth as sage Narayana, and how he had obtained boons from Shiva that made him invincible. It was Narayana who created Nara and protected him throughout the war. Earlier Shiva asked Jayadratha to spare Arjun as he was protected by Krishna. The Agneyastra was rendered ineffective due to Shiva’s boon to Krishna otherwise, it would have incinerated the entire Pandava army along with Arjuna and Krishna.
Literal plot armor was needed to stop this particular weapon. What more do you need to prove its power? Nothing in Mahabharat comes close to the power of this Astra which was empowered by Ashwathama'.. A’thama himself being a part of Rudra could be another reason for its immense destructive power.
Those two Rishis, said to be older than the oldest gods, take their births in every Yuga for serving the purposes of the world. Thyself also, O thou of great heart, hast been born as a portion of Rudra, by virtue of all thy religious acts and as a consequence of high ascetic austerities, endued with great energy and wrath.