I'm looking for a module I can (with some tweaking, obviously) drop into a pre-existing campaign. My players are level 11 with 1 mythic rank, but they're pretty well optimized so I could probably go for around CR 13 or 14 on average. It needs to be in a cold or frozen setting. Other than that I can probably work a lot in. Any suggestions?
There is a "By Region" section under the Adventure section (that I have only just noticed and deleted one of my own posts as a result)
There doesn't seem to be many at the levels you suggest in stand alone modules. The only one of high level is for 17th level in Irrisen
Have you looked at Reign of Winter Book 5? Set in 1918 Siberia.
Obviously a very crazy departure and you would need an explanation on how you jumped in time and planets. But that is the closest you will get I figure...
Its a little above the range you are looking for - but The witchwar legacy is a standalone module at 17th level set in Irrisen, and the one Lanathar mentions above.
Something like Academy of Secrets, which takes place 100% indoors, albeit with some handwaved traveling from one building to the next might work. (set for level 13 characters This index might help you know what might be found there...