Kinetically Augmented Ballistic Overkill Obliteration Machine (Patreon)

Weapon (greatsword), very rare

The line between genius as insanity is a blurry one. You gain a +2 bonus to attack and damage rolls made with this weapon. This weapon can hold up to 5 charges. You can cast a spell that deals fire damage into the engine of this weapon. The spell has no effect and this weapon gains a number of charges equal to the level of the spell slot used. You can instead insert a Red Corundum Elemental Gem into the engine of this weapon. As long as the engine holds the gem inside it, the weapon regains all charges after a long rest.

Booster Blade. As a bonus action you can spend a charge to fire up the engine of this weapon and supercharge your attacks. Until the end of your turn this weapon gains the thrown property (range 20/120 ft.) and your attacks with it are made with advantage, dealing an additional 1d8 fire damage on a hit. After each melee attack you make, you must succeed on a Strength saving throw where the DC is equal to 13 plus the number of attacks you have made this turn. On a failed save you lose your grip on this weapon as the rocket continues to fire and it flies a number of feet equal to 10 times the number of attacks you have made this turn in a random direction or until it hits a structure. Any creature in the weapon’s path must make a Dexterity saving throw where the DC is equal to 13 plus the number of attacks made this turn or take 2d6 bludgeoning damage.

Developed by a master weaponsmith long tired of smelting yet another boring blade, this contraption was only meant to be a project to stave off ennui. But even its creator could not have accounted for the power this hunk of metal and engineering would have on the battlefield, where the propulsive nature of its design allowed for it to batter through scores of foes who could barely see it coming before meeting their untimely demise. Unfortunately it's sheer destructive power made it incredibly unwieldy, deeming it too risky for mass development.