Under-king
Who is he? I'm rereading CC3 and he says he came from the land of the shifters, so Erilea. He crept through when the shifter fae came and he was in hiding there ("caves and barrows [he] was confined to"). Does that mean he's Valg? What else could he be?
Side note: I love that Hypaxia's reaction is basically, 'blah blah blah we didn't come here for a history lesson' 😆 juxtaposed to the reader's "omg so much lore/ Easter eggs give me more info Under-king!"🤯
Update: That was fast! A barrow wight. Thanks, everyone!