Underground

You're new to the UNDERGROUND, aren'tcha?

Flowey, after introducing himself

The Underground is the primary setting of Undertale. It is a large region within Mount Ebott that is separated from the Surface of the Earth. It houses the kingdom of Monsters, who are ruled by King Asgore, and formerly by his ex-wife, Toriel. The Underground has varying climates, and features six different areas: the Ruins, the frigid and wintery Snowdin, the marshy and cavernous Waterfall, the blistery and barren Hotland, the technologically advanced CORE and the Capital. The Underground's exit is blocked by the Barrier, which was placed by the Humans long ago. Anything and anyone can enter the Underground by descending into Mount Ebott.

History

View up from the Ruins where the first human fell.

After the Humans defeated the Monsters in The War, the losing faction was banished into the Underground. A Barrier was erected to prevent the insiders from leaving, though outsiders could still freely enter. Wary of humans, monsters retreated into the Ruins and adapted Home as their first capital, living normal lives in the Underground from then on. Additionally, the Ruins was structured to contain diversions in the form of puzzles.

At an unspecified point, monsters documented The War, regarding SOULs, and Delta Rune's prophecy on Waterfall's walls.

One day, the first human ascended Mt. Ebott and tripped on a vine, causing them to fall inside and enter the Underground. The human was discovered by Asriel Dreemurr, the son of Asgore and Toriel, and was adopted into their family. The first human's presence brought hope to monsters. Eventually, no longer fearing humans, monsters ventured outside of the Ruins, populated the rest of the Underground, and made New Home their new capital.

One of the areas in Hotland.

In contrast to Home, some monsters were content to settle at different locations than in New Home. From Waterfall, monsters acquired modern technology and discarded items from the Surface. At some point, the CORE was created by W. D. Gaster to convert electric power for the entirety of the Underground. Snowdin's blocks of ice is utilized to help keep the CORE from overheating.

However, one day, the first human grew ill and requested to see the flowers in their home village. After they died, Asriel absorbed their SOUL and became a powerful entity that was able to cross the Barrier. He carried their corpse to the center of the human's village, which had a patch of Golden Flowers. The humans attacked Asriel, under the assumption that he had killed the human child. Refusing to retaliate, he returned to the Underground with the first human's corpse and died in New Home's garden where he succumbed to his wounds.

One of the many puzzles found throughout the Underground.

Since then, Asgore became vengeful for his son and plotted to escape the Underground and declare another war against humans. He declared all humans who fall into the Underground 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 own while Toriel exiled herself into the Ruins. Over time, six more humans fell into the Underground, 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, the protagonist, had then fallen into the Underground. Unlike the predecessors, the protagonist's Determination carried them through many obstacles the Underground had presented, even managing to befriend or kill many monsters. Eventually, the protagonist manages to reach Asgore, where he stands before the Barrier. One of few events occur depending on the protagonist's actions:

• On the Neutral Route, the protagonist and Asgore battle. The latter is defeated, after which Flowey intervenes and absorbs the six human SOULs. The protagonist defeats Flowey and is somehow transported outside the Barrier after the event, where they leave the Underground. Sans then calls the protagonist and tells them the fate of the Underground, which is either peaceful or grim depending on who was killed.
• On the True Pacifist Route, Asgore is stopped by Toriel and all of the protagonist'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 Underground, reverting to his true form, Asriel. However, with all the SOULs, Asriel gains compassion and ultimately loses the will to fight, and instead he uses their power to shatter the Barrier. Having returned the SOULs, Asriel leaves, knowing that he will revert to Flowey, and begs the protagonist to forget him. The monsters are free to leave the Underground and live peaceful lives on the surface.
• On the Genocide Route, certain monsters try to stop the protagonist's murderous rampage while Alphys leads the evacuation. Upon reaching Asgore, the protagonist instantly defeats him with one blow and proceeds to destroy Flowey mercilessly. The first human appears before the protagonist and explains that their determination brought them back to life. With the protagonist's guidance, they realized that the true purpose of their reincarnation was to gain power. The first human then, with or without the protagonist's consent, uses that power to destroy the world.

Trivia

• The Eight Humans falling to the Underground might be a reference to Alice in Wonderland.