The Kuru king Santanu wished to tie the knot with a fisherwoman named Satyavati in a village close to Hastinapura.
However Satyavati's father agreed to give his daughter's hand to Santanu on one condition.
He demanded that Satyavati's son with Santanu should be instated as the successor to the throne of Hastinapura.
The throne of Hastinapura rightfully belonged to Bhishma as he was the son of Ganga the first wife of Santanu.
However Bhishma fulfilled the wish of the fisherman who wished to see his grandson as the ruler of Hastinapura.
Bhishma gave up his claim to the throne of Hastinapura and took the vow of celibacy in front of the fisherman and Santanu.
Santanu bestowed a befitting boon upon Bhishma overwhelmed with his pledge of celibacy.
As per Santanu's boon, Bhishma would be able to choose the time of his death as per will and remain invulnerable to death for generations.
In other words, Bhishma would be immortal and have control over his death.
Bhishma received the boon of a celestial weapon known as Praswapastra from the gods known as Vasus.
Bhishma fought an epic battle with his preceptor Parashurama in the battlefield of Kurukshetra.
However he was unable to defeat Parashurama with his own valour in the battle.
The eight gods known as Vasus were Bhishma's celestial brothers.
On witnessing Bhishma's inability to win the battle, Vasus approached him to give a lethal weapon known as Praswapastra which was unknown to Parashurama.
Bhishma decided to invoke Praswapastra on his bow but was stopped by sage Narada.
Upon Narada's pleading, Bhishma decided to withdraw Praswapastra from the battle.
Parashurama considered himself to be defeated since he had no answer to Bhishma's Praswapastra.
Thus Bhishma was able to prevail over Parashurama in the battle.
Hence Bhishma accomplished the unthinkable feat to defeat Parashurama in the battlefield.
Bhishma received the boon of self-willed death from his father Santanu and the boon of the weapon Praswapastra from Vasus.
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