Then how was Jayadrath killed?

In Mahabharata, it is said that the Pashupatastra obtained by Arjuna from Shiva is never used. Then how was Jayadrath killed? On the 14th day of war drona created a terrible formation of 3 vyuhas. Arjuna penetrated into the vyuha and killed the major number of army. But when arjuna approaching jayadratha he was obstructed by 6 maharathis in the kaurava army. How ever he managed to face those 6 warriors but sun is approaching the western hill to be set. In that situation Krishna created an illusion that sun was setted by using his chakra. By seeing that Jayadratha came out of his chariot and came before Arjuna to celebtare his defeat. Then Krishna removed his illusion and told Arjuna to kill Jayadratha. Arjuna took a sharp naracha arrow and did cut the head of Saindhava but Krishna told Arjuna make sure that the head should not fall on the ground. Arjuna is keeping jayadrarha's head to be in the sky with his arrows by continuing his fight with maharathi warriors. Then Arjuna asked krishna how much time that he should keep that head in the sky? Then Krishna replied Arjuna that Saindhava’s father took a boon from lord Shiva that “Who ever makes my son's head fall on ground that person's head should be made into pieces.” Make that head to fall in the lap of his father Vridhakshatra. He is in Samanthaka panchaka now. Then Arjuna for the first time in mahabharat war used Pashupatastra and made Jayadratha's head to fall in the lap of his father. He is doing meditation at that time. Suddenly by seeing the cutted head in his lap he thrown away that head on the ground and his head was made into pieces. In this incident Kriahna told the place where jayadratha's farher stayed. But Arjuna did not know the perfect place where he sat and meditating. So he used pashupatastra but if he had known his perfect place with in his eye sight Arjuna would have completed this task with normal arrow