Musaigen no Phantom World
Myriad Colors Phantom World

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Since there are 0 reviews for this light novel, I'll write one quickly (and very late in time, 2020) with few inevitable spoilers.
This light novel has 3 different universes set in 3 different volumes. This different universes end up having relation with each other at some point in the story (volume 2 to be specific).
The anime adapted the first volume, but had some changes out of the normal range we usually see in any adaptation of something, just like anime-only characters. As we watched in the anime, this first universe is all fun and happiness, something that not many people liked, even worst because KyoAni ... applied its usual "moefication" and made it Slice of Life genre. This universe has its own ending that might be considered the "happy" one or the best ending.
The second volume however, it's a completely unexpected twist of genres, showing a second universe with a surprisingly dark and story, main characters and antagonists way more powerful...absolutely different to how the first volume's development works. Right here the two universes connect and the story becomes a multidimensional plot with one goal.
I haven't read the third volume yet, but following the story of volume 2, it's the third and final universe, as well as the ending of the story.
If you have doubts about reading this light novel because of the disappointing anime adaptation, you won't waste your time reading volume 2 if you enjoy reading time traveling or multidimensional stories.