Forgotten weapon (the emergence of strong monsters, after some action players)
complication of late games
Can there be a forgotten weapon has led to the cataclysm? It may be super units have been created to destroy the sorcerers, dragons and other elite beings or to protect the sources of mana. After the cataclysm, they would be able to sleep and waiting the emergence of special conditions.
Such conditions may be - increasing strength magicians in the world, a diplomatic victory of the people or fallen, the use of high % of sources in the world of magic, or something else.
Further their behavior would be:
Attack on the won a diplomatic victory, or in other circumstances the awakening of the capture sources manna elements and turning them into a base for attacks. In any case, it would brighten up the late game.