Barrier

The Barrier is a strong magical force separating the Underground from the Surface. The humans put this in place after the War to keep the monsters trapped within the Underground and the rest of humanity safe on the Surface.

Any human can fall through the barrier on Mount Ebott into the Underground, leaving them in the Ruins, where Toriel generally finds them. However, only a being with the SOULs of a human and a monster can pass through from the Underground out to the Surface. The barrier can be broken, but seven human souls are required to do so. Only eight humans have ever fallen through the barrier, including Frisk.

Location
The Barrier is technically surrounding all of the Underground, but its exit is located in Asgore's Castle in New Home. This exit is sitting in between the room after the Throne Room and an unknown location somewhere on Mount Ebott, as seen after the Barrier is shattered and Frisk and their friends view the sunset or sunrise. It may be that this exit is located in a cave, which is depicted in the flashbacks of the War of Humans and Monsters's aftermath: monsters are gathered at the end of a large cavern and humans are gathered just outside, watching as seven mages create the Barrier.

Features
The Barrier's most notable feature is the six human SOULs, which Asgore stores in containers beneath the Barrier. These SOULs are kept to potentially break the barrier, as they are the only things that can rid of the magical seal and set all monsters free.

Appearance
The Barrier appears to be a completely white, endless abyss with constant shadows and breath-like sounds that, like most of the Underground, does not seem to abide to the laws of physics.

Post-War
Following the War of Humans and Monsters, the barrier is created by seven human mages, sealing the monsters underground forever. Seven SOULs are required to destroy it, but despite this the King and Queen still took care of the first human to fall into the Underground, Chara, believing them to be the Angel in the Prophecy of the Delta Rune.

Chara attempts to break the barrier by having their adoptive brother, Asriel, absorb their SOUL, cross the barrier, and collect six other human SOULs from their former village. However, Asriel does not take six other SOULs, instead letting himself be hurt by the humans almost to the point of death. Carrying Chara's body, Asriel begins to head back to the Underground and, as soon as he crosses the barrier, dies instantly.

After Asriel and Chara's deaths, six more humans fall into the Underground. These are killed by Asgore, who seeks to avenge his children and give the Underground hope. Their SOULs are collected and stored in small containers beneath the Barrier.

Neutral Route
Years later, an eighth human, Frisk, falls into the Underground. Having survived all the way to the end of their journey, Frisk fights Asgore at the barrier. If they spare him, Asgore tells Frisk of the events that took place after his children died, and suggests that Frisk stay with him and Toriel. Flowey then kills him, absorbs the human SOULs, and transforms into Omega/Photoshop Flowey.

Flowey attacks Frisk with everything he can, SAVING and LOADING their death but eventually, the six SOULs revolt and make him powerless. If Frisk spares him, Flowey becomes confused and runs away. Frisk somehow crosses the barrier with just their human SOUL, returning to an unknown location.

True Pacifist Route
After a Neutral Route has been completed, Asgore and Frisk prepare to fight at the barrier once more. Instead of proceeding in the fight like usual, Toriel intervenes and knocks Asgore away, greeting Frisk and apologizing to them for leaving them alone. One by one, Frisk's friends enter the Barrier, yelling for Asgore and Frisk to cease fighting. Soon, Papyrus reveals that he called everybody to the Barrier, with the exception of Toriel, by the instructions of a flower.

Flowey appears and announces his plans, but almost every monster in the Underground appears to encourage Frisk in their fight. Flowey, having acted in a facade that he was being beaten, reveals that this is what he wanted the entire time and absorbs everybody's SOULs, reverting to his final form: Asriel Dreemurr.

Asriel attempts to take Frisk's Determination, and after a long battle, is defeated by Frisk's endeavor to restore Asriel's SOUL after doing the same to their friends'. Finally understanding, Asriel reverts back to a child and uses the SOULs of all the monsters and the six humans to shatter the barrier. Both he and Frisk wake up; Asriel goes to Chara's grave, waiting to turn back into a flower, and Frisk wakes up in front of their friends, where they cross through where the barrier had been and head to the Surface.

Genocide Route
Frisk never crosses through the Barrier while on the Genocide Route, having killed both Asgore and Flowey before any of the Neutral Route events take place. Instead, Chara is fully resurrected and confronts Frisk, destroys the world, and takes their SOUL.

Geography
There is not much other than the magical barrier itself, with the exception of the small compartment below the Barrier that stores the containers keeping the six SOULs safe. Six out of seven of these containers are full, the seventh meant for Frisk's SOUL, but it is never used.