Theory series by Dorami, a skilled South Korean player and level creator in Geometry Dash, features several challenging levels that require a high level of skill and concentration to complete:

  1. The first installment, Theory of FirePower, draws inspiration from Theory of SkriLLex and is currently rated as a Hard Demon due to its demanding memory and timing requirements.
  2. The second level in the series, Theory of Insomnia, features F-777's remix of the song Insomnia by Mr. FijiWiji and presents players with another challenging Hard Demon level.
  3. Theory of IntercepT serves as the third level in the "Theory" series and presents players with another set of intense challenges and obstacles to overcome.
  4. Finally, the fourth and most difficult level in the series, Theory of XoanoN, features an extremely challenging gameplay experience that requires expert-level skills and precision. Despite its high level of difficulty, this level is visually stunning with its shiny Demon aesthetic.