Arjuna did a tremendous penance by standing only on one leg and concentrating his gaze upon the blazing sun. The deities became very impressed by Arjuna’s penance. They went to lord Shiva and requested him to bless Arjuna. Lord Shiva accepted their request.
Duryodhan had sent a demon named Mooka to kill Arjuna. Mooka had disguised himself as a boar. Arjuna was engrossed in his meditation, when suddenly his concentration got disturbed by a loud noise. He opened his eyes and saw Mooka being chased by a band of Kirats. In fact it was none other than Shiva who had appeared in the guise of Kirat.
Both Arjuna and Lord Shiva struck the boar with their respective arrows at the same time. As a result the boar was killed.
Lord Shiva sent his ganas to bring back the arrow. Similarly Arjuna too went near the dead boar to bring his arrow. Arjuna found his arrow lying on the ground. He picked it up in his hand. Right then the ganas arrived there and asked Arjuna to return that arrow as it belonged to their master (Shiva). But Arjuna refused to part with his arrow. Instead he asked the ganas, to convey his challenge to their master to have a dual with him.
The ganas went back to lord Shiva and narrated the whole story to him. Lord Shiva accepted Arjuna’s challenge and went to fight him. His ganas accompanied him. Arjuna defeated all the ganas of Shiva. In the end Lord Shiva came forward to have a dual fight with him. Shiva was very much impressed by his valour. He revealed his real identity. Arjuna was very ashamed that he fought with Shiva. Lord Shiva consoled him and gave his weapon Pashupat to Arjuna.