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The curse cast on the armies that tried to subdue the dark forces of the Frozen Kingdom was called the 'Undying Curse of Corruption'.
It was a curse that would slowly corrupt the minds and the bodies of the people or any creature that was affected by it.
Their minds were slowly getting corroded by it until they lost their sanity while also contaminating their bodies with its corrupted powers, slowly turning them into Corrupted Beings.
If not for the guardian dragon that was accompanying them, then Caelum and Nel would have already become mindless beasts just like the others. But despite that being the case, they still couldn't stop corruption in their bodies which still led them to become Corrupted Beings.
Draco and Lycan were somewhat of an exemption since they belonged to the group who were almost reaching the heights of a Divine Being.