Sword of Fire

The Sword of Fire is one of the Golden Weapons of Ninjago, formerly used by Kai. It appears as a golden katana with an elaborate, fire-breathing dragon head design on the hilt that extends up most of the blade.

Power
The Sword of Fire is able to engulf its blade in flames and shoot fireballs at enemies or objects. It can also transform into the Blade Cycle if the wielder can maintain their focus and concentraition.

History
The Sword of Fire was used by The First Master of Spinjitzu along with the other three weapons to make Ninjago.

Garmadon's Banishment
When the First Spinjitzu Master passed on, he entrusted the weapons to his two sons, Garmadon and Wu. However, Garmadon attempted to steal the weapons for his own selfish needs, forcing Wu to confront him before he could get away. After Wu defeated his brother and banished him to the Underworld, he hid the weapons.

Ninja Discovery
When Sensei Wu's four ninja proteges, Kai, Cole, Zane, and Jay were searching for the Sword of Fire, they decided to rest in the Forest of Tranquility after having found the other three weapons. In the middle of the night, Kai wakes up to see Nya standing nearby calling for him, which lures him to the Fire Temple.

Upon reaching the Temple, Nya is revealed to be Lord Garmadon's shadow in disguise, Lord Garmadon showing Kai the real Nya suspended by a chain over the lava. He tells Kai that the only way to save her is to cut the chain with the Sword of Fire, and despite knowing that he is playing into Lord Garmadon's trap, Kai pulled the sword out of its pedestal and saved Nya.

Kai is then faced with several shadow versions of himself, and while trying to fight back he realized that he cannot make any contact with them. Sensei Wu appeared using his own shadow to fight off the others.

Lord Garmadon awakened the Fire Dragon, and Sensei Wu departs to the Underworld in an attempt to keep the Golden Weapon from his brother.

In Samukai's Hands
Sensei Wu woke up in the Underworld after passing out during his journey. He then decided to go search for Lord Garmadon.

The Ninja recruited the four Dragons, using them to cross over to the Underworld, and after they used the Tornado of Creation to fend off the Skulkin and Spykor, they searched for Sensei Wu.

Sensei Wu found Lord Garmadon, who commanded Samukai to attack him. The Ninja arrived during Sensei Wu's battle, losing shortly after. Samukai wielded all four Golden Weapons at once, however, due to the power of the weapons being too much for him to handle all at once, he disintegrated and created a portal for Lord Garmadon to escape through.

In Defense of Ninjago
After Lord Garmadon escaped from the Underworld, each Ninja took up the weapon corresponding to their element, leaving Kai is possession of the Sword of Fire. The Fire Dragon, Flame, continued to guard it by becoming Kai's mount and pet.

Trivia

 * In some sets, it is called the "Golden Dragon Sword."
 * This was the last Golden Weapon of Spinjitzu to be collected by the Ninja.
 * In LEGO Universe, the Sword of Fire was simply a pair of Golden Katanas, similar to the Nunchuks of Lightning being in a set of two weapons as well.
 * It is the only Golden Weapon to have an identical appearance in the playsets and the show.