Helmet of Shadows

The Helmet of Shadows is a piece of headgear seen in 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 prophesized final battle.

History
The Helmet of Shadows' origin is unknown, but it likely belonged to The Overlord during his war against Ninjago and the First Spinjitzu Master. At some point, it was used to seal the Celestial Clock's countdown to the final battle.

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 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 Garmadon. (The Day Ninjago Stood Still)