Undertale

Undertale is a role-playing video game developed and published by Toby Fox.

Plot
Long ago, monsters and humans ruled the Earth together in peace. One day, war broke out, and after a hard and long-fought battle, the monsters were banished to the underground, a massive cave large enourgh to have its own weather system. They were sealed in by a magic barrier that was at the top of Mt. Ebott, created by the human's seven greatest magicians. Anyone can enter through the barrier, but only something with an incredibly powerful soul could make it out. You play as the human Frisk, who has fallen through the barrier, and strives to make their way out. The are many different monsters, who can be friendly or evil. You can make many friends, or you can kill them all. But be warned-if you kill to much, you may lose something that is impossible to get back...

More about the plot at Underground - Undertale Wiki

Characters

 * Alphys
 * Annoying Dog
 * Asgore Dreemurr
 * Asriel Dreemurr
 * Chara
 * Flowey
 * Frisk
 * Mettaton
 * Monster Kid
 * Muffet
 * Napstablook
 * Papyrus
 * Sans
 * Temmie
 * Toriel
 * Undyne

Het

 * Alphore - the ship between Alphys and Asgore Dreemurr
 * Asgoriel - the ship between Asgore Dreemurr and Toriel
 * Dogamyressa - the ship between Dogamy and Dogaressa
 * Dunkin Donuts - the ship between Sans and Muffet
 * Mettalphys - the ship between Mettaton and Alphys
 * Muffby - the ship between Muffet and Grillby
 * Muffeton - the ship between Muffet and Mettaton
 * Muffgore - the ship between Muffet and Asgore Dreemurr
 * Papyruffet - the ship between Papyrus and Muffet
 * Sansdyne - the ship between Sans and Undyne
 * Sanphys - the ship between Sans and Alphys
 * Soriel - the ship between Sans and Toriel
 * Undyrus - the ship between Undyne and Papyrus

Slash

 * Bones N Pellets - the ship between Sans and Flowey
 * Kingdings - the ship between Asgore Dreemurr and W. D. Gaster
 * Nicepants - the ship between Nice Cream Guy and Burgerpants
 * Papycest - the ship between a Papyrus and another Papyrus
 * Papyton - the ship between Papyrus and Mettaton
 * Sansby - the ship between Sans and Grillby
 * Sanscest - the ship between a Sans and another Sans
 * Sansgore - the ship between Sans and Asgore Dreemurr

Femslash

 * Alphyne - the ship between Undyne and Alphys
 * Bratcat - the ship between Bratty and Catty

Non-Binary

 * Annoying Tem - the ship between the Annoying Dog and Temmie
 * Chans - the ship between Chara and Sans
 * Chara Kid - the ship between Chara and Monster Kid
 * Charisk - the ship between Chara and Frisk
 * Chasriel - the ship between Chara and Asriel Dreemurr
 * Flowisk - the ship between Flowey and Frisk
 * Flowtem - the ship between Flowey and Temmie
 * Frans - the ship between Frisk and Sans
 * Frisemmie - the ship between Frisk and Temmie
 * Frisk Kid - the ship between Frisk and Monster Kid
 * Friskablook - the ship between Frisk and Napstablook
 * Friskriel - the ship between Frisk and Asriel Dreemurr
 * Gastara - the ship between W. D. Gaster and Chara
 * Papyrisk - the ship between Papyrus and Frisk

Familyship

 * Dreemurr Siblings - the ship between Chara and Asriel Dreemurr
 * Skelebros - the ship between Sans and Papyrus

Poly

 * Charfrans - the ship between Chara, Frisk, and Sans
 * Chariskriel - the ship between Chara, Frisk and Asriel Dreemurr

Cargo

 * Charalate - the ship between Chara and chocolate
 * Papyghetti - the ship between Papyrus and spaghetti
 * Sanschup - the ship between Sans and ketchup

Fanon
Undertale is a very popular game due to its unique design and long, creative story. Some people don't like it because it is "old school", but many fans believe that it is very unique, and almost nothing like an "old school" style RPG.

Shipping
Undertale fandom is also prone to multishipping and crack pairing fic and art.

The fandom is known for being largely involved in anti-ship movements, groups of people who attack other fans over their shipping preferences. A lot of these reported cases center around Frisk, Chara, and Asriel due to their ages and the fact that people write smut about them.