Arcanic Arm Cannon, Darkness Flame

This cylindrical device slides over a medium humanoid hand with plenty of room to make the necessary somatic component for Eldritch bast. The device warps your spell into a demonic black flame blast that burns your targets while you are attuned to it. After a short rest you can choose between this version of the Arm Cannon or another one to use until your next short rest.

When you cast Eldritch Blast through Arcanic Arm Cannon you fire a single blast with one attack roll instead of individual beams but on a successful hit you roll all the D10s for your spell caster level.   

Below 5th caster level you deal an extra d8 of fire damage on a hit and the target must make a successful Con save against your Spell Save DC or be Set On Fire taking 1d4 necrotic damage until they spend an action to put out the flames.

At 5th level you deal the extra fire damage and you can select a target within 5 feet of your original target to take the same amount of damage on a failed Dex save ( your spell Save DC) or half as much on a successful one. The original target is the only one  that makes the Con Save

At 11th level you can affect 2 additional targets within 5 feet of your original target on a failed Dex save ( your spell Save DC) or take half as much on a successful one. Alternatively you can concentrate the beam to deal an additional d10 instead of a d8 to a single target. The original target is the only one  that makes the Con Save

At 17th level the extra force damage becomes a d10 and you can affect 3 additional targets within 5 feet of your original target or you can focus the beam for a d12 ,instead of a d10,to a single target. The original target is the only one  that makes the Con Save

Notes: Eldritch Blast