Complex sentence: According to the myth narrated to this event, "asuras" (demons) sent a boar to kill Arjuna, when Shiva appeared on the scene to protect Arjuna assuming the form of "kirata" (hunter).
Split into simpler sentences: According to the myth narrated to this event, "asuras" (demons) sent a boar to kill Arjuna. Then Shiva appeared on the scene to protect Arjuna assuming the form of "kirata" (hunter).