Both Undertale and Deltarune are so mixed together and in the same moment, they are worlds apart. The Delta Rune's storyline is about Kris, who lives in the monster town with Toriel, one of the central characters of the original game. What is even more strange is the fact that with the help of the first episode, Toby Fox, questions the audience, tries to understand what they want to see next. But he released Deltarune a full-fledged first chapter with 2-3 hours of gameplay with a finished plot, working mechanics, and even a secret boss. No one expected new revelations from Toby Fox, the author of Undertale. A strange and complex little game, with a ton of references to the classic JRPGs, unpretentious graphics, and a fabulous storyline, was well welcomed by both critics and players. The Undertale and Deltarune popularity is hard to explain.
