Once, Arjuna was performing a tremendous penance to please lord Shiva. 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 Keerat. In fact it was none other than Shiva who had appeared in the guise of Keerat. Both Arjuna and Lord Shiva struck the boar with their respective arrows at the same time.
A disagreement broke between Arjuna and Kirat (Shiva’s avatar) over who killed the boar. Arjuna challenged Shiva for a dual. Shiva was very much impressed by his valour. He revealed his real identity. Arjuna was very ashamed that he fought with Shiva but Lord Shiva consoled him and gave his weapon Pashupat to Arjuna.