You Walked Right Into Our Trap
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Jon was the only one that remained within Arthur's parent's manor, as his parents were evacuated into the underground area that connected both manors.
Using Noah's Vampiric Eyes of Hypnosis and Stupefaction, Noah sweet-talked the parents into obeying him and spending the night in these areas, without actually revealing that the Vampire family was waiting for their enemies to arrive.
Afterward, Noah left several Blood Servants with Jon, who used Clairvoyance to see through almost the entire territory of the manor while closing his eyes and then using Far Talk and Telepathy to communicate to Noah what was happening.