Helmet of Shadows

The Helmet of Shadows is a piece of headgear seen in LEGO Ninjago: Masters of Spinjitzu. It gives the wearer control over the Stone Army, and serves as the seal for the Celestial Clock - if it is removed from its resting place, the clock will begin counting down to the prophesied final battle.

The Day Ninjago Stood Still
When Lord Garmadon arrived on the Island of Darkness, the Overlord guided him to the Celestial Clock and encouraged him to take the Helmet of Shadows. When Lord Garmadon put the helmet on his head, the hands of the Celestial Clock began moving once again, and the Overlord explained the significance of the artifacts to Lord Garmadon.

The Last Hope
Seeking a way to delay the final battle, Misako suggested to the Ninja that the Celestial Clock could be halted if the Helmet of Shadows was returned to its pedestal. Sneaking into Garmadon's camp as a prisoner "captured" by Stone Warriors (actually the Ninja in disguise), Misako managed to convince Garmadon to remove the helmet long enough to steal it and make her escape in the Power Drill. As the Stone Army gave chase, Jay considered wearing the helmet, but was discouraged by the others, who suspected that its dark powers would corrupt any non-evil wearer.

When the Ninja reached the Celestial Clock, they searched for the helmet's pedestal. Unfamiliar with the clock's design, they were delayed long enough for General Kozu to arrive and reclaim the helmet. It changed hands repeatedly in the resulting scuffle, only for Misako to finally spot the pedestal. Unfortunately, she replaced the helmet a second too late to stop the countdown, forcing the Ninja to fall back as the Stone Army kidnapped Nya.

Return of the Overlord
The Stone Army returned the Helmet of Shadows to Lord Garmadon, along with the captured Nya. Garmadon was wearing the Helmet once again when he began piloting the Garmatron to attack Ninjago. When the Overlord possessed Garmadon's body, he continued to wear the Helmet.

Rise of the Spinjitzu Master
The Helmet of Shadows fell from the Overlord's head during his metamorphosis into his true form, and the villain left it in the streets of Ninjago City. This would prove fortunate for the heroes, as the Helmet would be donned by Dareth, and Misako was able to explain its function in time for the self-proclaimed "Brown Ninja" to save the Ninja from the Stone Army. Dareth then used his new headgear to send the Stone Army against the Overlord, buying time for the Ninja to begin ascending the dark dragon's tower.

Trivia

 * The Helmet of Shadows appears in the set 70505 Temple of Light however, it is not mentioned on the box that that is what it is.
 * At a few points in the series, it is hinted that the Helmet of Shadows may be able to infect its wearer with darkness. So far, this has not been proven.