The Pashupatastra (IAST: Pāśupatāstra), in Hindu History, is an irresistible and most destructive personal weapon of Shiva, Kali and Adi Para Sakthi discharged by the mind, the eyes, words, or a bow. Never to be used against lesser enemies or by lesser warriors, the Pashupatastra is capable of destroying creation and vanquishing all beings. Pashupatastra is the weapon of Pashupatinath, the most important of all Shiva temples, located in Kathmandu, Nepal.Maha Kameswara Astra, the personal weapon of Adi Para Sakthi is the weapons that can neutralize the Pashupatastra.Lord Shiva obtained Pashupatastra from Adi Para Sakthi by intense penance before the creation of the Brahmand.
In the Mahabharata Arjuna obtained this weapon from Lord Shiva but did not use it because this weapon would destroy the entire world, if used against a mortal enemy. Arjuna used this astra to slay Jayadaratha.It is said that the mantra to obtain and discharge the astra is sealed by Shiva to prevent its misuse in the Kali Yuga. It is said that no one in the three worlds can resist lord Shiva when he shows his prowess. Apart from Arjuna,Sage Parashurama and Ravan's son Indrajit no other warrior possessed this weapon. It is also said that when indrajith used the pasupathastra against laxman it doesn't harm him because Laxhamn was the part incarnate of Lord Vishnu.
A Narrative from KMG translation of Mahabharat regarding the power of Pashupatastra:
O thou of mighty arms, that weapon(Pashupatastra) is superior to the Brahma, the Narayana, the Aindra, the Agneya, and the Varuna weapons. Verily, it is capable of neutralising every other weapon in the universe. It was with that weapon that the illustrious Mahadeva had in days of yore, burnt and consumed in a moment the triple city of the Asuras. With the greatest ease, O Govinda, Mahadeva, using that single arrow, achieved that feat. That weapon, shot by Mahadeva's arms, can, without doubt consume in half the time taken up by a twinkling of the eyes the entire universe with all its mobile and immobile creatures. In the universe there is no being including even the deities, that are incapable of being slain by that weapon.