This is an upgraded version of the Winter Prowler- something that visits and takes lives every winter as sacrifices. Even worse, nobody has yet managed to track down this winter menace let alone kill it. Could be anything from some kind of wild animal that has got good camouflage abilities in winter and can sneak in and out of places quietly/nimbly to some malicious ethereal beings.
29. Communication from a frontier military outpost far from the centre of the Kingdom has been cut off during an especially cold winter. Investigators sent out report back that the outpost has been encased in a dome made of unknown material. A potential reason for this: the outpost was built on top of a ruin from an ancient civilisation that had set up a Dome of Warmth spell to be triggered when the temperature falls under a given threshold. In particular, the spell had become dysfunctional over the years and had locked all living beings within the vicinity of the Dome (when originally, it was supposed to be a protective measure against especially cold weather by providing an area of warmth for inhabitants enclosed within but still leaving them freedom to exit when the need arose)
Instead of being limited to a given section of town, the Winter Illness seems to affect people randomly within an entire Kingdom. The cause? A new or previously undiscovered poison is at play. In particular, this poison can hibernate within a human’s body until the internal temperature falls below a given threshold.
(note: this is partially inspired by the Friday freetext of poison)