Werewolf Chess is orthodox Chess with the Queen replaced by a similar, but contageous piece

Contagion is a game mechanic that first appeared in Maka Dai Dai Shogi, a large Shogi variant created by Japanese Buddhist monks in the Edo period. It is one of the methods to ensure the most powerful piece(s) cannot be easily traded out of the game. The idea is that when you capture a contageous piece, (in this case the Werewolf), the capturing piece instantly promotes to (i.e. changes into) a Werewolf.

This profoundly changes the way tactics works, and ensures that after an exchange involving Werewolfs a Werewolf will remain. An attempt to trade Werewolfs will thus only lead to disappearance of one of the Werewolfs, strongly discouraging the player that would be left without one to engage in such a trade.

To make the King not entirely defenseless against a Werewolf, he is immune to the contagion, and can thus safely capture a Werewolf. This will provide a way for all Werewolfs to disappear, but only when the King participates in tactic. (So usually not before the end-game.)