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Leagues are temporary content that can only be played on specific realms for a span of 3 months and later determined to be core. They are often if not always side story from main content

Expansion are large scale content patches that applies to the whole game as instant core and furthers the storyline/lore of the game

I do understand that marketing and hype wise it’s better to say “expansion” but it’s still not the truth

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over 4 years ago - /u/Bex_GGG - Direct link

We consider them expansions because they include changes that affect the core game - like new skills, unique items, divination cards, balance changes and other game reworks. Also, more often than not league content rolls into the core game.