@Overly Serious
Now magic and all, but for all practical purposes generic Undead should freeze solid once the temperature drops below freezing due to no way to warm their flesh.
But that could be part of the terror - South Pole crossing, except if you take too long to cross the undead will thaw out and eat your face.
And that gave me an idea abotu comboing that with the Whale Hunting; Captain Baha hunting the undead Wight Whale.
There's your monster hunter campaign.
"From the Nine Hells, I stab at thee..."
Heh. Repurposing the classics of literature is fun. I've done that on rare occasions and whilst I've never concealed my source I've also never lampshaded it. I like leaving it to the players to notice. Sadly, they rarely do. Uncultured things!
And yes, you could certainly have the undead affected by the cold. I'd imagine zombies to be more affected as they must have moisture in their flesh and seem to depend on muscles for movement, than the dry bones of skeletons that don't necessarily even have ligaments (though I would describe them as still having some, or maybe bones bound together by wound twine -details like that always add some atmosphere). Very dessicated zombies would appear like mummies, probably.
But as an aside, I've never liked using zombies. I prefer something that is more clearly animated by magic like skeletons or if it has physiological processes, that the dark sorcery is something more powerful. Frankly, it's always felt to me like skeletons should be the
more dangerous than zombies - faster, more magical, with senses that you don't understand.
On the other hand, finding a field of corpses standing up where they froze, thinking you just lucked out in avoiding a zombie horde, and then the bodies break up so the skeletons can chase you around could be a fun little setpiece.
That's a great image. Heroes picking their way through an area of sleeping / unaware threats is a classic for a reason. Put something at the centre that the PCs have to get to - a hut, a fort, a fortified cave entrance. Have it that the skeletons were besieging it before they froze or that those inside couldn't get out and eventually died causing the skeletons to cease. Now it's a forest of the damned that PCs have to go gently through. If they fuck up, or maybe if they do something wrong at the target place (riddle, spell, trap...) they'll hit the snap of ice cracking all around them as the skeletons begin to move. Could have them start slow due to ice but getting faster, or simply more breaking free every round - get out of there before the number hits critical mass and they're freeing themselves faster than you can kill them.
Or go with ghouls as
@Ghostse mentioned. Ghouls have always been a bit ambiguous over if they're actually 'living' undead or not. Seeing them sleeping, frozen in the ice and rimmed with frost. When they wake up they will be starving.
Ghouls might be best thematically as one of the themes of tundra should be starvation. It's not for nothing so many horrors of the region are things like the cannibal Wendigo.
Some examples I can think of are things like the Bukovac, the Walker in the Winds, ancestral spirits gone wild, and things like the Giant Mosquito that turns into a swarm of them when you kill them.
Yes - exactly these sort of things are appropriate. If the PCs have hirelings or they're caravan guards or something, then you can use monsters that possess people. Maybe a wendigo spirit is stalking them, invisible and ethereal until at night they find someone killed and partially eaten and they have to figure out who is possessed. Maybe a new person every night.
I'd argue with no flesh to freeze skeletons would be mobile but anything low-tier and fleshy would be frozen solid unless it was a special frost zombie. Something like a Ghoul could be argued to have some sort of metabolism and you could play it they are either frozen in hibernation with the zombies or they are able to generate enough heat due to rampant hunger (but would eventually "burn out" if unable to feed)
Yes - this would be very creepy. I like how a tundra setting is immediately making us all think of a Horror vibe.
What are they actually going to be doing in this tundra? If they're capable of planar travel we need some way of making sure they're not just teleporting over this endless terrain. The vibe of tundra is
distance.