Indie Cross Fnf 〈5000+ EXTENDED〉
Here is the fan theory accepted by most: Indie Cross occurs inside a "Corrupted Dream" that Boyfriend has after touching the Ink Machine. This explains why the backgrounds glitch and why the music distorts when you lose.
However, a word of caution: . If you have not finished the Genocide route of Undertale , the mod's story twist will completely ruin the original game for you. indie cross fnf