Legacy of the Void, the third episode of StarCraft 2's story, will introduce a brand-new way to play the game called Archon Mode.
Archon Mode allows two players to work together in order to control a single base and army. The intention is to provide both a new twist on the game's competitive multiplayer component, and also allow less experienced players to enjoy the heat of battle with a little extra guidance. There'll be new multiplayer units for all three of StarCraft 2's races as well.
If you've not yet got around to playing through either Wings of Liberty or Heart of the Swarm, the previous StarCraft 2 episodes, then have no fear. Legacy of the Void will be released as a standalone title, and won't require the previous two games to play.