The mission must first be activated by a guild member with "Mission Control" permission. A minimum of 3 guild members currently representing the guild must then participate in an event.
There are 3 levels, each with different requirements and rewards.
Summon 2 Mist Champions. Time limit: 30 minutes. Recommended: 3 players.
Win 2 PvP Stronghold Matches. Time limit: 40 minutes. Recommended: 5 players.
Summon 18 Mist Champions. Time limit: 55 minutes. Recommended: 8 players.
There is no "reset" time after abandoning these missions. The missions can be launched again immediately.