Chaos Inside The Manor
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The Demonic Energy that Cassandra unleashed were not only to get Ellergest and Blake away from her sight, but also with the intention of taking care of the people inside the manor! She wanted them all to either become Lesser Demons or be eaten by them. She couldn't simply allow them to survive after everything that happened, and she planned to just discard this entire family, grabbing her child in the process, and then running away when she was done with the priest and this annoying child.
She knew her child would be okay because he had been born after she underwent the demon ritual, and her son was a special child capable of wielding demonic powers and that was born with them. Naturally immune to most negative effects that demonic energy had.