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. Two to three 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 Nebula Arcanum can consume mana very rapidly under constant use. Players should have items that provide mana at the ready when using this item, such as the Mana Flower and Mana Regeneration Potions.
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 any other pre-Moon Lord magic weapon, but has difficulty striking fast enemies and bosses.
If attempting to clear the Celestial 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 do not move and have enormous hitboxes, allowing all of the effects of the weapon to deal full damage.
The Nebula Arcanum's damage output is high enough that it can be used to skip most of the Empress of Light's attack rotations, making it much easier to fight during the daytime.
The orb held by the player when using the weapon.
When using the Nebula Arcanum, the player holds a dark purple orb orbited by a pink ring. The projectiles appear to emerge from this orb.
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 item's inventory sprite resembles typical depictions of a crystal ball, which are often wrapped in cloth for safekeeping.
The Nebula Arcanum greatly resembles the Arcane Orb from the hack-and-slash action role-playing game Diablo III in both appearance and function.