Everquest. The sheer amont of Lore that has been created for Norrath is astounding. Only the Forgotten Realms and Greyhawk worlds from Dungeons & Dragons had more lore, and those were table top games. I am not sure what qualifies as unique, EQ is pretty much high fantasy but all of it was created by those early developers. Actually, some of the main characters in EQ (Lucan D'Lere) were actually their characters in their own D&D home world that they ported over to the EQ Norrath world.
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The Chronicles of Spellborn... for its time.
Though The Secret World deserves kudos as well.