The sky arcanum let’s you manipulate air, lightning, and can manipulate your breathing. The moon arcanum is illusion magic. Sun is fire and light. Earth let’s you manipulate plants and well… earth and helps promote growth. Ocean is water and ice. So what would the star arcanum be? It seems like most magic you’d think of has been covered by another arcanum. The only hints we really have are that a unicorn horn is necessary for the spell that killed Avizandum, and Aaravos says to viren that he’s messing with powerful “Life and death magic” when talking about infantis sanguine. However that spell I’d assume is dark magic since Claudia and viren would be casting the spell and neither are connected to an arcanum.