Depretale is an UNDERTALE AU in which every character is depressed except Frisk, Chara, Asriel, and Gaster.
About the AU
The cause of all these monsters being depressed is a gas which is being expelled from the Garbage Dump. The only person able to survive there w/o getting more depressed is Napstablook, so the gas can't be cleaned up. Chara cannot reset this timeline, which leaves resetting to Frisk, Asriel, and Sans.
Frisk was trying to cheer everybody up. Gaster was brought back to life in exchange for the life of Burgerpants, who killed himself by jumping into the CORE because he thought his job was too depressing. Gaster found a way to make a portal that would last forever in exchange for his power to reset, but the portal is glitchy and brought Chara and Asriel back to life. Some of the monsters left through the portal, but eventually, they all came back except Asriel. Chara is started murdering all the monsters, and the only way to stop her was for Frisk to reset the timeline. Chara was fighting Sans right before the reset, but Sans escaped to a different timeline to save himself. Asriel wasn't erased from where he was, as he was in a timeline other than Depretale.
After the reset, Frisk convinces Alphys to block off the Garbage Dump. Alphys then invents a way to connect to the Wi-Fi on the surface and download anime illegally. Gaster, Chara, and Asriel are not brought back to life post-reset. Asriel from before the reset finds that Gaster's portal is gone and decides to make his own portal to an off shoot timeline of the one he came from, in which Chara killed everyone then got depressed and jumped into the CORE. This offshoot timeline is called the Orbiscide timeline. Sans found the Orbiscide timeline and decided to live there inside Grillby's.