Oni Mask Arc

The Oni Mask Arc is one of the story arcs in the series that coincieds with the Twelve Portals Arc.
It is because of this arc that Diane became the first Shadow Guardian.
== Synopsis ==
To increase security around the True Heir of Meridian, Wong attempts to summon the Shadowkhan. Instead he unleashes an ancient evil known as Tarakudo, Lord of all Oni and King of the Shadowkhan. 
The Guardians find out that the masks have been scattered across the globe and that each mask contains the power to control a different tribe of Shadowkhan as well as a different ingredient to remove and subdue the mask.