The Great Door East

* The Great Door East is the door that links the throne room to the outside world. It remains under Mt. Ebott and prevents Monster to leave the Underground cavern.

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* This is the door that possess the magical barrier and need to be open to free the monsters. The seal on it was created by seven human wizards after the war and requires seven human souls and a monster one to destroy it permanently. However, a monster soul with a human sou or a human with a monster soull can cross it but this would be a single passage, and not a permanent opening.

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* Asriel, after the death of his adoptive sibling, absorbed Chara's soul and crossed the barrier to bring the latest's body back to their village. But the humans attacked him and Asriel/Chara went back to the Underground to die, before he could collect six human souls to break the barrier permanently.

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* The Great Door East works in harmony with the Great Door West, that remains at the beginning of the cavern, in the Ruins, and opens on Mt. Ebott pit, in where humans fall down.

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* The magic of the Doors is a "one way door": it only allows people to cross from one side but not the other. There is a few other small doors working the same way in the Underground, such as the Forest Door.