The souls plays entirely on the Deathless mechanic that this jam runs off of. The PCs collect Soul fire from monsters and can use it to bring themselves back from the dead. They also start with 1 from my understanding. As for monsters, that is indeed meant to be filled by the GM on the mountain itself. I would combine it with my Silent Night taking place on the mountain for a stalking horror that they could theoretically lure in to sacrifice.
The Cold and Deathless Northern Darkness Jam » Entries » Conquest of Cthulhu: An Adventure for Mörk Borg