Missions are events that reset each Cube World day. They can be found via crossed swords on the map. Currently, missions involve defeating a boss monster or a dungeon boss. At any time, there are three (3) missions available to complete.
Once a player is close to a mission's area, a tooltip will pop up, telling the adventurer to "Defeat the ruler at <location>", where <location> can be any generated area. The area can either be an open-area dungeon (like a forest, huge rock, etc., or they can be dungeons, like castles or pyramids. Another type of mission is completed by defeating a boss monster. In this case, the tooltip will display, "Defeat <boss>", where <boss> is any boss monster's name.
Note: Taming the boss monster of a mission does not complete the mission and renders it impossible to finish.
Missions are a feature that will be expanded upon, other such tasks than killing bosses will become available later in development.
Missions can be infinitely reset and selected by talking to Innkeeper inside the Inn of a city. When you reset the time, the missions and quests change, and castles, dungeons, and bosses reset. You can also "select" missions by continually resetting the time until the crossed sword icon appears above the location you would like. This is also a good method to find portals.