He forced himself and summoned a giant demon head, its purpose is to be eaten just like Nightmare, he has to feed him a constant amount of energy to preserve his life.
The two girls were now panicking inwardly, they tried to make the tentacled monster vomit Nightmare out but unfortunately for them, their strength is too weak to successfully achieve this mission.
"You dare eat my stead!" Sans voice rang throughout the battlefield, the two girls were at first startled before celebrating, Leafinia quickly arrives next to Kuros and helps him preserve Nightmare's life.
The situation is not looking good for him, the opponent they are currently facing is an Outer World Saint Lord, a being that is born to devour and absorb countless energy, Nightmare would certainly not last long.
