AtG Apparatus

This strange apparatus is a rectangular black box with two ever-dark holes lengthways into it, mounted on a hinged arm which easily mounts to any armor or a shoulder.

Whenever you score a critical hit, the apparatus activates, launching a glowing fiery missile at a random hostile creature within line of sight.
The missile impacts and deals 4d6 Fire damage and 2d6 Thunder damage as it detonates.

Notes: Damage: Fire, Damage: Thunder, Damage, Combat