DeNA has announced that the Legendary Arena: Cobalion has been added and the related missions will be available form July 6th, 2020 to July 20th, 2020.

Legendary Arena: Cobalion Newly Added!

Legendary Arena: Cobalion Available Now! Legendary Arena: Cobalion is a brand new battle area that has just been added today! You can complete the related missions to earn Medals that can only be obtained here!

Date/Time

July 6, 2020 at 11:00 p.m. - July 20, 2020 at 10:59 p.m.

Legendary Arena Details

The Legendary Arena is a special battle area where you can form multiple teams and take on the challenge using a set number of sync pairs. Each difficulty level has a maximum number of sync pairs that can participate. Sync pairs that have already participated cannot do so again until the challenge has been reset.Strategically form teams for each Legendary Arena challenge and take on the boss!

Missions

You can earn Medals and other completion rewards that are only available during this Legendary Arena challenge period!

Note: For more details about missions, tap the Mission Completion Rewards button displayed on the Legendary Arena difficulty selection screen.

Important

• To attempt Legendary Arena missions, you must have completed Main Story Chapter 2:Defeat Team Break!(Normal)in single-player.
• To learn more details about the Legendary Arena, tap the Information button on the main Legendary Arena screen.
• While Legendary Arena missions will also be displayed on the General missions screen, they cannot be attempted outside of the challenge period.
• The time period and featured content are subject to change without notice.

Source: https://pokemonmasters-game.com/en-US/announcements/Event_2060_2W_1