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The flavor text describes it as an Aura. So it would seem that swarms would be weak to it. However, it doesn't have the Aura trait like other Aura effects do.

Book 1 Mantle of Gold, Chapter 3, Tolorr Family Festival, Phase 1: Guests Arrive / Phase 2: An Unwelcome Entrance, Page 59-60.

Phase 1 says "Guests: All guests arrive"
Phase 2 says "All guests are now present as Ygrin Oathcarver arrives."

It seems like the characters are meant to notice that Ygrin has a negative response from the crowd by having him arrive separately. So Phase 1 should probably say something like,

"Guests: All guests arrive except for Ygrin Oathcarver."