Undertale

Undertale is a roleplaying 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

Fandom
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.

Het

 * Asgoriel - the ship between Asgore and Toriel
 * Dogamyressa - the ship between Dogamy and Dogaressa
 * Sanphys - the ship between Sans and Alphys
 * Soriel - the ship between Sans and Toriel
 * Undyrus - the ship between Undyne and Papyrus

Slash

 * Fontcest - the ship between Sans and Papyrus
 * Papyton - the ship between Papyrus and Mettaton
 * Sansby - the ship between Sans and Grillby

Femslash

 * Alphyne - the ship between Undyne and Alphys

Non-Binary

 * Blookyton - the ship between Nabstablook and Mettaton
 * Chans - the ship between Chara and Sans
 * Charisk - the ship between Chara and Frisk
 * Frara - the ship between Chara and Frisk
 * 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
 * Frisriel - the ship between Frisk and Asriel
 * Gastara - the ship between Gaster and Chara
 * Papyrisk - the ship between Papyrus and Frisk

Familyship

 * Chasriel - the ship between Chara and Asriel
 * Skelebros - the ship between Sans and Papyrus