Death and Reincarnation.
Kuroe Makoto, the tomboyish ace of her high school track and field club, never spared a thought for these things before. However, when she falls asleep in her Tokyo home after an exhausting marathon and wakes up in a fantasy world, those concepts become her reality. And unlike in the common light novel setting, she isn't reincarnated as the naturally good-looking and talented human hero who gathers a harem effortlessly.
In fact, she isn't reincarnated as a human at all. She is reborn as 'Chaos', child of the demon queen and heir to a kingdom at war with humanity.
Maou Shoujo Magical Chaos puts the Japanese high school girl in a tentacle monster, in this strange tale of a sound human mind possessing a horrifying creature's body. Join Chaos on her oftentimes humorous but also perilous journey through a world of sword and sorcery, as she discovers a newfound appetite for life, terror, and demi-human girls.
This is the web novel version, essentially a first draft. A much more polished version with a lot more content is available on Amazon under the same name.
Cover art by Peperon.