Seize the special weekly bonus that gives you exclusive access to the Land of Demons by raiding Mystic Land dungeons!

Go to [Soul Training] > [Weekly Rewards] in the bottom left to see the Weekly Rewards. 
Weekly Rewards reset every Monday at 5AM.

Every time a specific dungeon is raided in the Mystic Land, you can open the Weekly Rewards, which contains that enables you to exclusively access Land of Demons.
You can choose a Mystic Land that includes a Weekly Reward, and since each Mystic Land has different rules for the Land of Demons exclusive entry buff so choose accordingly.