Myth: The Rat King Expansion Boss

From the publisher's webpage:

Few have spoken of the monstrous abomination known as The Rat King. It is said his body is a teeming mass of diseased tailless continuously swarming over his monstrous body, protecting him. This protection is an illusion though as the Rat King is a powerful assassin able to destroy those foolish enough to break apart the swarm. Born of the shadows, the Rat King is a master in the art of becoming invisible and seeing those who dare hide in the dark. Those who face the Rat King should expect to also face an endless swarm of Tailless as they rush to their King’s aid.