Underworld Helmet

The Helmet of Garmadon, also known as the Underworld Helmet, was a helmet that acted as a symbol of authority. Before Garmadon put on the Helmet of Shadows, he left behind his helmet next to the Celestial Clock. This helmet held many ancient evil powers, which grew as they sat, stagnantly. Anyone who wears the helmet will be corrupted, as the helmet serves as an infinite source of bubbling Corruption! also known as Dark Matter. The helmet can corrupt anything that enters in its bowl-like bosom, including weapons.

History
Once fallen warriors began to populate the Underworld as undead Skulkin, the Underworld Helmet was put to use as a symbol of authority amongst the skeletons, with Samukai eventually arriving and seizing it for his own.

An Underworldly Takeover
With Garmadon banished to the Underworld, the disgraced hero sought a means of escaping the realm by taking control of the Skulkin, and after defeating a vengeful Samukai in combat, donned the Underworld Helmet for his own.

With the symbol of Skulkin authority now his, Garmadon would proceed to command the skeletal beings, wearing it even after betraying and forsaking the fallen warriors.

The Day Ninjago Stood Still
Offered the Helmet of Shadows by the Overlord in order to begin the countdown of the Celestial Clock, Garmadon accepted the being's offer, discarding the Underworld Helmet in favor of the Stone Army controlling artifact.

Trivia

 * In the sets, the Underworld Helmet comes in two forms; The first in the 2011 sets, being comprised of the traditional Samurai helmet with the small bone piece, and later the 2012 edition, with the helmet now replaced by the new, narrower helmet used in the Samurai X helmet.

Movie Version
In the movie, Garmadon has a new helmet, that will be in show in Season 8.