Rat kings

Rat kings exude a psychic aura of terror. Every turn, its enemies need to make a DC 13 Wis save or become frightened of the rat king until the start of their next turn. It has stats similar to a cranium rat swarm but does not have resistance to slashing damage. Cutting the knot of tails in the center (with a slashing critical hit) will kill the swarm intelligence of the king. If it still has hit points, the remaining rats will continue to fight as a swarm.