The Auroral Knight
Arceius wears a special armor created for him by the order who trained him. When it's innate magic is activated it causes the peculiar Aurora like effect that earned him his title upon graduation. Not particularly strong, the armor is made of lightly tempered steel and it's true purpose is to augment, direct, and focus Arceius' various supernatural abilities.
The sword he wears on his back, Sky's Tear, was made for him by the insane Void Kissed warrior that brought Myradin into the struggle initially. It is pure carbon, bound by magic to make it stronger and denser than any normal sword, but is otherwise unremarkable. It has a large amount of sentimental value but bears no sign of use due to it's magical protections.
Arceius casts a form of magic that he brought with him from his old universe, Suune's Magic. Instead of forming his casting into specific pathways to produce more structured results, Arceius merely gathers energy from the Aether (an energy field that infuses space-time in Places that have been opened to magic) and flings it at people. While unsophisticated, this ability can be extremely versatile to those with sufficient ingenuity - though it is very limited in scope.
It is important to note that Arceius' magic barely effective against any being that has an aura and that his own aura protects him similarly. This is called Suune's Blessing and only alters the effects of Suune's magic. If the subject is sapient, sentient, or an object incredible importance (personal, cultural, or symbolic) it has an aura. Even seemingly overwhelming attacks against anything with an aura will product little to no result as long as the source of this damage came from Suune's Magic. It's incredibly inefficient and nearly impossible to kill or maim someone using this form of magic, and there is no way to go around it. Exceptions to this general rule include people incredibly adept at Suune's Magic (who are still bound by Suune's magic in their treatment of others) though you'd be hard pressed to harm them in any case, and people who have willingly opened themselves to the magic being used (however this only allows you to use what magic they expect you to use on them, if they open themselves up to allow you to heal them you cannot harm them with the magic instead: Suune's Blessing will still intervene).
Arceius' Basic Rotation Magic works as follows:
Once a powerful warrior Arceius used his power to drive away a world destroying entity. While he tried putting his past of violence and bloodshed behind him Myradin tried to move on, but the call of his own brand of justice always draws him back. Arceius feels the the past is best left where it is, hoping for a new start in the new places he finds himself but he has three important remnants left over from his life in Shämuhl. The magic he uses, Suune's Magic; the imprisoned Zultheriel, Shadow of Tsevaresh locked away in his Mind; and the dangerous demonic infection known as Thessily, held in check for so long by powers from The Old World in Shämuhl.