How did the greatest archer Arjuna got killed by his own Son

Uloopi Restoring Arjuna's Life through
the Nag Mani.

Arjuna married Chitrangada the princess of Manipur during his exile and they both had a son named Babruvahana, but Arjuna had to leave her wife and newborn son back to Manipur because Chitrangada's father The King Of Manipur did not have a son whom he could announce as his Crown Prince, so he took an oath from Arjuna that Babruvahana would not fight in the holy war to ensure that Manipur didn't loose its only Prince in the war.
    After The Holy War of Mahabharata Yudhishthir was crowned as the King of Aryavarta (Present-day India). 

He conducted Ahsvamedh Yagna, Where a decorated horse would be sent across the kingdom and wherever it goes unopposed, the land would be acquired by the king who sent it. 

    When Arjuna went to Manipura with the wandering sacrificial horse of the Ashvamedh, Babruvahana captured the horse, which, by tradition, meant war against the Pandavas. Arjuna tried to persuade Babruvahana to leave the horse as there was no enmity between Manipura and Hastinapura. Babruvahana agreed but with one condition that he would like to defeat Arjuna in a battle duel for his Guru Dakshina.  Arjuna, reluctant to fight a young boy, left and informed a small troop of his army to convince Babruvahana to return the horse . Babruvahana defeated the army. He also defeated Bheema. Arjuna got enraged and got in a battle with Babruvahana. Babruvahana killed his father Arjuna with an arrow, a boon given to him by Ganga, Bhishma's mother. When Babruvahan came to know the Warrior whom he had just killed was his own Father, after knowing this Babruvahana was determined to kill himself, but he was intervened from his stepmother, the Naga princess Uloopi, a gem called Nagamani which restored Arjuna to life with the help of Krishna. Arjuna couldn't believe that his Son was such a fierce warrior and at the same moment, he was pride too. Arjuna admired Babruvahana's skill in war. Then, the Pandavas, Uloopi, Chitrangada, Babruvahana, and the armies returned to Hastinapur.