Legendary Journey is unlocked at lvl 55 together with the Wish Star Platforms.
This game mode is like a "Story Mode" where you need specific Pokémon to defeat some maps and complete the story. Once you complete one Legendary Journey, you can simply raid it daily for a chance of getting the following Rewards: Ancient Silver, Ancient Gold and Wish Stars.
There's a total of 22 different Legendary Journeys available, each one providing their own Wish Star. When completing or raiding a Legendary Journey, it is not guaranteed that the Wish Star will drop; usually the Wish Stars have a low Drop Chance.
If you are on our Discord Server, you can go to the #pokeland-help channel and use the help.wishstar command so that you know what are the wish stars available for each Legendary Journey:
Legendary Journey also has a Hard mode which is much harder to complete. Here you can obtain items for Dynamax, and every time that you raid one Hard Journey you have a chance to encounter a Wish Boss.
Wish Bosses will drop materials that will be used for Wish Plates, which is something much harder to grind in comparison to the Wish Stars.