XEL
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In the third game, there was an invasion of the Conflux (castle) from another dimension into the world of Enroth.
Let's say, in the expansion, the appearance of a race from a parallel world (Enroth) in the world of Ashan – due to unsuccessful experiments in the field of magic or a deliberate struggle for the throne – would be a good plot twist.
The castle should be practically unchanged in terms of its creatures since the third game. The series' continuity is restored.
These could be the elves of Rampart, the mages of Tower, or the creatures of Fortress.
The appearance of the dwarves or elves from the fifth game in the expansion would also be welcome.