Underground

"" The setting of the vast majority of Undertale, the  is a large region underneath the Surface of the Earth, which houses the kingdom of Monsters. Exit from the is blocked by the Barrier, which was placed by the Humans ages ago. It is accessed by humans, the protagonist included, by way of a hole in Mount Ebott. It is ruled by King Asgore, and formerly by his ex-wife as well, Toriel. Spanning several different climates, the features the cold and wintery Snowdin, the wet and cavernous Waterfall, the blistery and barren Hotland, the technologically advanced Core, as well as the Capital.

History
After the Humans defeated the Monsters in The War, the losing faction was banished into the. A Barrier was erected to prevent the insiders from leaving, though outsiders could still freely enter. The monsters eventually accepted their loss and lived normal lives throughout the.

One day, a human named Chara passed through the Barrier and entered the. Chara was discovered by Asriel, the son of Asgore and Toriel, and was adopted into their family. However, the human one day grew ill and eventually died from the illness, but wished to see the flowers in their home village. After Chara died, Asriel absorbed their SOUL, becoming a powerful entity that was able to cross the barrier, and picked up their corpse and carried it out of the. However, the humans from the outside world attacked Asriel, under the assumption that he killed Chara, forcing him to return to the with Chara's corpse where he succumbed to his wounds.

Since then, Asgore became vengeful for his son and plotted to escape the and declare another war against humans. He declared all humans who fall into the be killed and their SOULs collected; with seven human SOULs, the Barrier could be shattered. However, the brutality of this decision disgusted Toriel enough that she relinquished her position as queen as well as her marriage to Asgore.

Asgore continued to rule the land on his owns, while Toriel exiled herself into the Ruins. Over time, six more humans fell into the, all of which Toriel tried to shelter and care for. However, they all eventually left the Ruins and were eventually killed, their SOULs collected by Asgore.

An eighth human, Frisk, had then fallen into the. Unlike the predecessors, Frisk's Determination managed to carry them through many obstacles the had presented, even managing to befriend or make an enemy out of many monsters. Eventually, Frisk manages to reach Asgore, where he stands before the Barrier. One of few events occur depending on Frisk's actions:


 * In the Neutral Route, Frisk and Asgore battle. The latter is defeated but is intervened by Flowey, who kills Asgore, and then absorbs the six human SOULs. Frisk defeats Flowey and is somehow transported outside the Barrier after the event, where they leave the . Sans then calls Frisk and tells them the fate of the, which is either peaceful or grim depending on who was killed.
 * In the True Pacifist Route, Asgore is stopped by Toriel and all of Frisk's friends, but are then ambushed by Flowey, who proceeds to absorb the six human SOULs as well as the SOULs of all the monsters in the, reverting to his true form, Asriel. However, with all the SOULs Asriel gains compassion and ultimately loses the will to fight, instead using their power to shatter the Barrier. Having return the SOULs, Asriel leaves knowing that he will revert to Flowey and begs Frisk to forget him, while the monsters are free to leave the and live peaceful lives on the surface.
 * In the Genocide Route, certain monsters try to stop Frisk's murderous rampage while Alphys leads the evacuation. Upon reaching Asgore, Frisk instantly defeats him with one blow, and proceeds to mercilessly destroy Flowey. It is then revealed that Frisk is under the complete control of Chara, revealing their true sinister nature, and proceeds to use Frisk to leave the and destroy the world.

Trivia

 * The Eight Humans falling in Underground may be a reference from Alice in Wonderland.

地下世界