Okay, may as well start with this, those guards cannot beat Kelly's fascinate

Spoiler: Dorni
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So first things first, this is definitely not a natural phenomenon, whilst portals to other realms can occasionally open on their own, it's very unlikely to happen around here what with Avalon hogging the local leylines. So that only leaves active and possibly malicious intent, the number of people who could pull this off on a whim is fairly slim, being limited to mages of Merlin's calibre like the Archmage or Black Erzebet or truly powerful reality benders, those sorts of people definitely wouldn't be put off patrols of trainee heroes.