This Thief type Trap Room was introduced in Boss Monster: The Next Level
This card is meant mostly for breaking Treasure stalemates with an opponent.
Example: The player and an opponent have an equal amount of Mage Room treasures. Upon activating Decapitator, the player can choose an opponent's Mage Room and remove its treasure value, thus bringing in any Mage Heroes that have been left in town.
However, the effect of this card can also remove the treasure value of a card in the player's own dungeon, thus sending all the Heroes to the opponents dungeon. This strategy works well after the player has deactivated an opponent's room by means of Freeze or Frostbat Cave. This also could work as a dangerous combo when Epic Heroes are in play, and cause an opponent major damage.
This, along with a few other Thief Trap Room cards, seem to have a pattern of being like killer products, ending with "-nator". Two other rooms like this include The Crushinator and the Smashinator.