Arthur defeating a superior stage grandmaster once but not landing the killing blow was partially acceptable. Zach was monsterized, and not many knew he still retained his sanity so they assumed he was unable to fight properly, unlike Owen.
However, doing it a second time, to a monsterized devil who was capable of switching in and out of his monsterized form at will, one who obviously retained his sanity.
People were bound to become suspicious, throwing in the rumours of him being a grandmaster and the gods who has been ignoring him for a while began to take action again.
Most of them didn't dare send spies into Cattleya's territory, but the few who were bold enough to do so, they received a few 'gifts' from Cattleya.
Packages containing the spies they sent to find out if Arthur was really unable to use his energy.
'Air freight, and in very small boxes. Ha! Mom's as ruthless as ever.'
