to make motivation for finding the real Canta Slaus, the townspeople could just not believe your claims that this Canta Slaus isn't the real one, and Canta Slaus would use it to his advantage in trying to remove you guys from the picture.
then once you get the real canta slaus back, you guys realize as the townspeople are also in on this, as the fake one has promised to give them power within their cult.
then triple twist on your players as they learn that Canta Slaus was kept alive as the seed for the rebirth of the Winter God, and the only way to stop him from turning is to kill him, if the sacrifices are completed, and stopping the last needed sacrifice turns out to be very very difficult as the town has been fortified while you were away finding Canta Slaus. Canta Slaus wouldn't mind taking one for the world, but expresses lament at all the people that would go without gifts and the loss of yet another powerful good force in the world. morally ambiguous scenarios for the win