The Nebula Arcanum is a Hardmode, post-Lunatic Cultistmagic weapon that fires a multicolored swirling orb when used. The orb will slowly home in on enemies and bounce off tiles, hitting three times before exploding into several smaller projectiles. All three hits do not have to be on the same target, as it will continue to home on other enemies in range if its target dies before the third hit. If the projectile does not hit three times, it will explode after 11 seconds. It also produces additional pink projectiles that orbit it and damage multiple enemies, even going through tiles to do so. 2-3 of these projectiles appear per second for the first half of the projectile's lifetime, and disappear at the same rate as it approaches explosion.
The lasting duration, homing properties, and orbiting projectiles make the Nebula Arcanum a very effective crowd-clearing weapon.
The Nebula Arcanum is capable of dealing more DPS than its other Nebula counterpart, the Nebula Blaze, but has difficulty striking fast enemies and bosses.
If attempting to clear the Lunar Pillars as a magic user; the Nebula Arcanum can be incredibly effective at destroying each of the pillars when their shields are down, as they don't move and have enormous hitboxes, allowing all of the effects of the weapon to deal full damage.
Trivia
The projectile shares a sprite with the Lunar Portal as well as the portals that appear in the Vortex area, although they are not the same color.
The Nebula Arcanum greatly resembles the Arcane Orb from the Diablo trilogy in both appearance and function.