This is a work of Lord Shiva and Parvati, disguised as tribals (hunters), purely to test the essence of Arjuna's penance. In deep meditation of Shiva to obtain the most destructive, the most powerful weapon -- the Pashupatastra -- Arjuna is unshakeable in his determination. Shiva decides to test his resolve and takes the guise of a hunter and just then a demon in the form of a boar attacks Arjuna. Both Arjuna and Shiva fire their arrows killing the animal (demon) instantly: while Arjuna fires from the front, Shiva hits the animal from the back. In some stories, it's said Arjuna mocks Shiva for doing so, little realising who he is talking to. Legend has it that they decide to fight each other to settle the argument and at one point, Arjuna grabs the hunter's feet in a bid to topple him which proves enough for Shiva to present himself in his true form. A shocked Arjuna instantly prostrates before the lord and Parvati and is blessed with the weapon that he so desired.