Arcanamech Arm

Mechanical Prosthetic: This arm only functions of affixed to the shoulder to replace a missing or amputated arm. Alternatively, it can be adjusted to attach at the elbow. Then you gain access to the following benefits. 
Arcane Crystals: You gain a +2 to your spell attack modifier and you ignore half cover when making a spell attack with this arm. 
Power Surge: The arm holds 10 charges. Before making a weapon or spell attack roll you can expend any number of charges and add that number to your attack roll. The arm regains 1d10 charges after you finish a long rest. 

Notes: Bonus: Spell Attacks, Arcane Class