The Truce (comic by Jakei95)

THIS COMIC IS NOT BY ME. Its creator took it off of her tumblr page and has declared it non-canon to her other works (Underverse and X-Tale), so I wanted to preserve it here because it's still really good (and also because there are some elements that are still canon and because it's a good introduction to the whole Undertale AU network in general).
This comic takes place in the fan-made multiverse related to the video game Undertale, and contains several characters not made by this comic's creator (Ink!Sans by Comyet/Myebi, Error!Sans by Crayon Queen, and CORE!Frisk by DokuDoki). It is also a prequel to the comic X-Tale, which takes place in an Alternate Universe that Jakei did create, as well as her Alternate Universe crossover animation series that follows X-Tale, which is called Underverse. All of these (although sometimes hard to follow) are worth reading/watching for any Undertale fan.
If you want to learn more about some of these characters' backstories that aren't explained in this comic, start here https://undertale-au.wikia.com/wiki/Undertale_AU_Wiki or here https://undertale-au-fanon.wikia.com/wiki/Undertale_AU_Fanon_Wiki (these websites also contain spoilers for X-Tale and Underverse, so if you plan to read/watch those, then don't go to those pages)