 Mukasura went to the forest where Arjuna was practicing his vows of prayer, vigil, and fast and attacked Arjuna in the form of a boar.Arjuna was actually conducting his penance to please Siva and obtain the great weapon called "pasupatha-astra" (the missile of pasupatha, the lord of the animate world).Lord Siva sends Mukasara in the form of a boar to test the severity of Arjuna's penances.