Once upon an ancient time, there was a land known as Etheria. Etheria was under the tyrannical rule of a nasty race of beings known as the Old Ones. 5 legendary heroes (The Knight, Grand Magus, The Elven Archer, The Chi-Master, and The Messenger) came together and created The Etherian Council Of Defense. When (Cthulu) The MoonLord and his wife The Empress Of Light saw the Council's uprise, they had a change of heart and decided to withdraw their presence from the war. Unfortunately, one of the 5 legendary heroes had ignored the Council's warnings about the danger of being a double agent without a partner (or any sort of backup support at all), and become power corrupt. He banished MoonLord and Empress Of Light to the closely connected land of Terraria, taking their place as the "Harbinger". The other 4 legendary founders hadn't come this far from never encountering betrayal before, and banished the old Ones into the Crystalline dimension.

A decade or so later, the 4 legendary founder's kids accidentally broke an Etheria crystal, (while the 4 founders were on a "crusade to the Crystalline Dimension that Harbinger was trapped in) unleashing the Harbinger and starting a mass collect-a-thon for the crystals. Terraria meanwhile, had a heroic champion of its own (a player guided by the Guide, MoonLord & Empress' son), similarly defeating bosses and ultimately freeing all the Terrarians from MoonLord's rule. Etheria's second-generation heroes, on the other hand, weren't so lucky. They became corrupted from personal nightmares (that occurred in a timerift) and became Harbinger's puppets.

The Jester (Harbinger's daughter), The Abyss King (Abyss Lord's [ruler of the Crystalline Dimension] son), and the King Prince (last remaining/alive heir) Of Dragonfall (kingdom/refuge) are holding the council together, keeping the 3rd generation of Etheria's hero(lings) alive. Now it's up to them and Terraria's new champion(lings) to help fix the chaos resulting from the Old Ones' reign.