The Thunder Sword is the starting weapon of GEO in the GEO Adventures though eventually replaced by GEOs Holy Weapon Mjölnir.
Whats it do? Edit
The Thunder Sword has a joybuzzer sweetspot attached to it so it has some shocking capacity it has 3 counterparts: the Wind sword and the Fire sword.
Its stronger, but less accurate then its steel counterpart it works off the users magic stat so potentially it could be a very good weapon.