Werewolf Den is a dual type Monster Room introduced in Boss Monster: The Next Level
This card works similarly to Dark Heroes, as a player can discard a card to give a Hero in another dungeon a boost in their health, but it only works for the player using this card. If another player has this card in their dungeon, and both players use this card on the same Hero, the effects stack, thus giving the chosen Hero +2 health.
Though, in a certain situation, it could also be given to a Hero in the user's own dungeon that, by chance, will die in a Room that has a "If a Hero dies in this room" effect. But this is highly situational.
This card makes a reference to the game Golden Axe, in which the protagonist has the ability to transform into a werewolf. This is evidenced by the fact the werewolf on the platform is holding a golden axe.
A couple excerpts from a couple updates featuring Werewolf Den:
"Werewolf Den is what we call a "cycling" Room, which allows you to spend an unwanted card for an interesting effect: give a Hero in an opponent's dungeon a lycanthropic boost!"
"Since you'll tend to have more Rooms in your hand during a game of Boss Monster 2, we wanted to give you more ways to use unwanted Rooms. Werewolf Den does just that, allowing you to "bite" a Hero to make them more deadly for an opponent."