My first thought was that it was an unforeseen consequence of the combination of Summer and Winter, causing Arcadia to breakdown, and the approaching figure was the King or Queen. I'm not sure that checks out though, and I have no idea what's going on. We know that the Wild Hunt fae who are with the army weren't able to open gates out, so the hellscape thing has been going on for awhile, but Cat just opened a gate not too far from the army and the part of Arcadia she traveled through at that point looked fine.
There was a fairly convincing argument in the comments that it's Zeze (Hellgates and something in the sky, indicating Flying Fortress-City, and possibly absorbing the power of Witnessing the events at that city), but I've got no idea. There aren't a whole lot of people who are capable of breaking off a splinter of Arcadia, and he's certainly powerful and crazy enough, but why would he be targeting the Army of Callow? I'm sure it's not just the location they happen to be at because they haven't been standing still. Maybe he's mad at Cat, and figured she would show up there eventually.
It doesn't seem to be the Dead King's M.O. but it's not like this isn't something he could do. Sure, he typically only attacks with massive armies of undead, but I remember Cat theorizing that he never actually led his armies in the previous invasions, so he might use all kinds of new tricks. He would probably be interested in a distracting battle that weakens all his enemies.
My theory? Larat or whatever his name is became/was-corrupted-by a demon, and now HellMirror!Arcadia is following him around.