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. Unlike common light novel settings, 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. 
The Tentacle Awakens puts the Japanese high school girl in a tentacle monster in this strange tale of a sound human who mind-possesses 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.
The Demon Princess Magical Chaos series contains gore and explicit sexual content. It's intended for mature listeners.