Ketone

Set in two worlds. One world is a world you know today, full of tehcnology, separated into seven different continents.
The other world is much less modern, a world of Spirit, Fire, Water, Wind, Earth and the Centre, a world of witchcraft and magic.
A Tailed Beast, Witches and Humans. The tailed beast known as the Ketone was a ten tailed monster, created by witchcraft and magic. The ten tails held the power of nine other animals and combined made a perfect weapon until the sealing spell of the witches was discovered and made humans immortal.