The Stress Monster is a manifestation of the person's stress.
The Stress Monster is a black entity that was made from the person's extreme stress. The Monster will emerge when the person has used an ancient bell to rid their stress out, forming their stress into a dark creature. They can also absorb other's personal stress forming more heads and granting full view of the area.
The Stress Monster can be damaged or destroyed with a assortment of weapons, but it seems that blunt weapons such as the Hammer were ineffective against the Stress Monster as we see Benson having trouble to properly damage the creature only for Pops' to give him duel Tomahawks that finally destroy his stress self.
His stress leaves his body and turns into a dark monster. He then runs and lunges at the stress and engages in battle with it. Rigby notices that Skips forgot his ninja sword and runs to grab it.
Everyone uncovers their ears in order to stop Rigby and they all float up high into the air. The stress leaves their bodies and goes into Skips' stress head, forming stress heads of Pops, Mordecai, Rigby, and Benson. Skips gets the sword and attacks it. Mordecai, Rigby, Benson and Pops help Skips battle it with the dead people's weapons.
The four then go after their own stress heads. They begin slicing off their heads. Benson faces difficulty with his stress head until Pops throws him two tomahawks which Benson throws at his stress heads' eyes, finally killing it. Now it was all up to Skips and his stress head. The four then try to help Skips again but the Skips stress head hurls them away, only wanting to battle Skips one on one. Skips then proceeds to spear it many times but his stress outsmarts him and traps him.
Skips then finds the willpower to resist from his stress' tentacles and spots a spear on the ground. He cunningly runs towards it, with his stress head behind him, ready to engulf Skips. Skips quickly grabs the spear and points it at his persistent stress head just in time as it almost engulfs him. The spears goes right through its mouth. Acknowledging the fatal blow, the stress head looks at the spear and is then killed.