Gas-Weapon

A weapon that is able to build up pressurized gas by striking your foes down, and able to use built up pressure to enhance your weapon.

Build Pressure: Upon making a successful attack roll you build 1 stack of Pressure, on critical attacks build 2 stacks.
Gas-Heating: As a bonus action, you are able to expend 3 stacks of Pressure to heat your weapon up and make it glow a hot red and orange. Attacks with this weapon deal an additional 2d4 Fire Damage until the end of your next turn.
Implosion: When you strike a creature with this weapon, as a bonus action you are able to expend 5 stacks of Pressure to overheat your weapon, causing the gas chamber within the weapon to implode. Dealing an additional 2d4 Bludgeoning + 2d4 Fire Damage.
Self-Implosion: At 10 stacks of Pressure the weapon automatically implodes in your hands, making you take the damage dealt by Implosion.
Release Pressure: As a bonus action, you are able to release up to 4 stacks of pressure at once. All pressure is automatically released if you have not made a successful attack roll on a creature in 10 minutes.

Rarity

Rare Magic Weapon (Any Melee Weapon that deals Slashing or Piercing Damage and is made with metal, Requires Attunement)