An arm cannon embedded in Zion's arm(s), specially designed by Alcyone Delta/Shizuni Subaru. It uses Zion's power source to shoot special energy bullets so as to not expend ammunition. The Zion Buster was created by using information that Delta/Shizuni was able to access by analyzing Zion's "blood" and her own personal knowledge on nanomachines.
Once per round, Zion can use a charge and fire a large blast of energy from his chargeable arm cannon at a single target. The target must make a DC 16 Dexterity saving throw. A target takes 4d8 radiant damage on a failed save, or half as much damage on a successful one. The buster has 5 charges and regains expended charges daily per long rest.
As a bonus action, Zion can retract the arm cannon into his forearm and transform his hand back to normal or extend it from there. While it is extended, he can use the weapon as if he was holding it, and he can't use that hand for other purposes.
It’s up to you to decide whether a character has proficiency with a firearm. Characters in most D&D worlds wouldn’t have such proficiency. During their downtime, characters can use the training rules in the Player’s Handbook to acquire proficiency, assuming that they have enough ammunition to keep the weapons working while mastering their use.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Vex. If you hit a creature with this weapon and deal damage to the creature, you have Advantage on your next attack roll against that creature before the end of your next turn.
Notes: Damage: Radiant, Zion and Mendicant Bias, Range, Ammunition (Firearms), Reload, Vex