KMS 1.2.145 - Union Revamp

Can someone tell me what Union is supposed to refer to?

During the last patch for the Legend update (the one after Demon Slayers), they added it in. Basically, all classes come together to fight the Black Magician. You get a free medal (500 HP/MP) and a beginner skill that gives 5 all stats 5 attack/magic attack.

Then this new update (Advance of the Union) expands on it, giving it some new quests and equipment.