The Legends Pass is a new Monster Legends system that allows you to obtain rewards and perks by completing challenges.
It starts on the first day of every month and is active for 28 days.

There are two types of Passes:
  • The Legends Pass
  • The Golden Legends Pass.
Both provide rewards but the Golden Legends Pass gives some very cool perks that are not available on the Legends Pass.

More details are available here