Thunder Bolt can refer to one of two weapons, both created by Lord Garmadon.
The original Thunder Bolt is a long, light gray staff. It appears to have Lightning emitting from the tip, but it is unclear if it is actual electricity or just a blade shaped to resemble lightning and charged with electrical power. It is Lord Garmadon's favored weapon, as it enhances his already-formidable powers. Despite this, it has never appeared in the cartoon series, being relegated to promotional spots and various playsets.
This weapon wasn’t made for fighting, it was built for smiting. Only one exists, and it’s in the foul hands of its creator, Lord Garmadon. It boosts his Power of Destruction, and makes him happy.
Golden Thunder Bolt
The Golden Thunder Bolt (also known as the Lightning Bolt Dagger) is an upgraded version of the Thunder Bolt weapon. It appears as a small golden sword or dagger with the same lightning/lightning-shaped blade as the Thunder Bolt.
It has even more power than the original weapon, despite being much smaller, and it would continue to be used as Lord Garmadon's preferred weapon. It eventually came into Lloyd's possession, although whether this is the same weapon or a replica made specifically for him is unknown.
Ninjago.com Description (2011)
Lord Garmadon has made a new and terribly-awesome weapon, the Golden Thunder Bolt. Unlike the non-golden version, this one truly makes him happy... 'cause now, it's got extra smiting power.
Ninjago.com Description (2012, renamed the Lightning Bolt Dagger)
Forged of the strongest underworldly steel, hardened and charged by lightning to make it diamond tough and electrifying, allowing you to zap or stab anyone who gets in your way. Lloyd's weapon of choice!
- This weapon's tip element is used in several other themes, including, but not limited to, Lego Star Wars (as Emperor Palpatine's Force lightning), Lego Mixels (as the tip of Teslo's tail and the top of the lightning rod on Zaptor's head) and Lego Marvel Superheroes (as Electro's lightning).