The Permafrost Ritual
The Permafrost Ritual is an ancient ritual that only members of the Hellsberg line know. Every year on the winter solstice rumor has it that the King of Kelton sacrifices a pure virgin and repeats a magical ritual that preserves the enchantment. In reality that is only half right.
Every winter solstice the barrier between the Realm of the Undead and the Realm of the Living is at its weakest. It enables the Hellsbergs to communicate with Balzathoun, a higher level demon, together they make a deal, an exchange of ten virgin souls every year in return for the cold.
However there is a third part that the Hellsbergs do not know about. Every time this ritual is performed, it weakens the barrier between the realms more and more. With the added strength of the virgin souls Balzathoun would soon be able to break through into the Realm of the Living.