Jet Hammer (3.5e Equipment)
From D&D Wiki
The jet hammer is a massive mallet from an alternate dimension. Made to look like wood (but is usually made of steel), it contains a small jet engine used for massively powered attacks. It takes 2 full actions to charge the engine (with a button on the side) so it can be used; when launched, the hammer opens up, revealing a flaming turbine. Those struck by the engine (it can hit everyone within 5 feet) take 3d6 damage (L), 3d10 (H) 3d20 (G), or 2d100 (C) in addition to the base damage. Afterwards, the user must rest for 1 full action. During this time, the weapon's shell closes. While charging, the user is immobile and has their AC reduced by 6, to a bare minimum of 0.