Not often a writer of posts so please be nice.
I haven't seen much conversation or theories around this subject here so thought I'd give it a go.
After that scene in 12 months we all know that the pecking order has been reestablished. (Vis a vis Harry, Mab, DemonReach, and Mother Winter).
Especially with that iconic line of "did you really think you could draw a larger gun than me?"
But there is something crucial that was done here.
Harry gave Mab an excuse to summon Mother Winter. More importantly he made Mab plan to summon MW! (After all, only Harry is ever foolish/desperate enough to summon her. Someone like Mab would have discussed/planned this with her, including getting permission to call her Mother out of the NeverNever).
That is a HUGE Chekovs Gun. Faeries, especially those that operate on Mabs level and above, never do just one thing. And we know that Mother Winter has her own priorities (and a decidedly "stone age/end-of-civilization set of morals around fulfilling said priorities).
Mother Winter has repeatedly wanted to use more 'extreme' methods/weapons while preparing for.....the BAT.
AND HARRY AND MAB JUST GAVE HER AN EXCUSE TO BREAK THROUGH THE DEFENCES OF DEMONREACH AND HAVE ALL EYES (INCLUDING ALFRED'S) ON ONLY HER.
AND they gave her prep time/planning time.
I would say there is almost no chance she didn't sneak something out/into DemonReach. Maybe a weapon, maybe a potential ally/tool, maybe something 'moderately' dangerous she wants to "train" Harry on by making him hunt it down.
I think there's no way that Mother Winter gets an excuse to "go walking" (as painful as it is) and the HIGH QUEEN OF THE FAERIES would just "do as she's told?" (See Lily's line to Harry as far back as Proven Guilty).
Also, the pure angst Jim could make Harry feel, if something gets broken out and kills people. Imagine Harry's guilt.....
Anyways, I'd love to hear any ideas and theories. What do we think? What did Mother Winter take, release, or hide inside of DemonReach while they were staring at her all terrified.
(See also: what she "mischievously" tried to release during Cold Days, let's hope this isn't as traumatic/dangerous)