Pneuma Blade

Weapon (longsword or greatsword), rare
  This blade shouldn’t work; the pneumatic seals are made of earwax and leather, the sprockets’ teeth are all different sizes, and the structure itself is held together by an adhesive tape decorated with small ducks. The only explanation is that the sheer force of the kobold mechamagicians’ belief is encapsulated within this magic sword, allowing it to chomp at its enemies.
  Pneumatic Strike. After you make an attack roll with this weapon, but before the result of the attack is determined, you can use a bonus action to pull the blade’s trigger. With a pneumatic thrust, the top of the weapon arcs forward and you gain a +4 bonus to the attack roll.
  Tetanus Blade. When you hit a creature with an attack that uses Pneumatic Strike, the pathogens that live within the rusty blade are reinvigorated by its magic, and the attack deals an additional 1d8 poison damage.
  Backfire. When you roll an 8 on Tetanus Blade’s poison damage, dormant pathogens within surrounding creatures become reinvigorated. Each creature within 10 feet of you (including you) must succeed on a DC 15 Constitution saving throw or take 4d6 poison damage and become poisoned until the end of its next turn.
  Very rare variant: With a refined pneumatic mechanism, this upgraded version of the pneuma pneuma blade is totally, indisputably, and inarguably, 98.4385% guaranteed not to backfire. The new-fangled heating element is also almost absolutely perfect.
  Increase the attack bonus of Pneumatic Strike to + 5. Change the Tetanus Blade to Heated weapon, this property deals 2d8 fire damage instead of the 1d8 poison damage. Change the Backfire to:
  Backfire. When you use the Pneumatic Strike property of this weapon and roll 8 on both of the additional fire damage dice, the heating mechanism backfires. Each creature within 10 feet of you (including you) must succeed on a DC 16 Dexterity saving throw or take 4d6 fire damage as scalding pneumatic fluid bursts out.