Kingdom Hearts Union X is the long awaited multiplayer update/sequel to Kingdom Hearts Unchained x.
Union X continues the story from Unchained x, part of which was turned in a cinematic for Kingdom Hearts 2.8: Final Chapter Prologue, and will be important to the story of Kingdom Hearts 3.
All gameplay from Unchained x is still present, and save data will be transferred over, while new players can still make their own avatars and choose their faction. Battles are also still the same, continuing the use a card selecting mechanic with special medals representing different Disney characters.
The main new feature Union X introduces is a multiplayer option. Players are now able to join with up to five members of their party and take on quests and battle Raid Bosses.
Union X is the second game in the Kingdom Hearts franchise to feature a multiplayer option, the first of which was 2009's Kingdom Hearts 358/2 Days for the Nintendo DS, and the first to feature online multiplayer.
This may be a sign Kingdom Hearts 3 may have a multiplayer option as well, but for now this is what fans will have to play to tide themselves over.