The Barrier

The Barrier is the magic wall in Undertale that separates the Underground, a large region beneath the surface of the Earth, from the world above, keeping monsters trapped there after an ancient war with humans. Asgore collected human souls in order to break it, and Alphys researched how souls could do so. In the True Pacifist route Asriel destroys the barrier after his fight with the human, and the monsters return to the surface.