Isshindōtai

Isshindōtai (一心同体,One in Mind and Body) is a ancient spell known to be able to give birth to a powerful entity.

Overview
Isshindōtai is known to be a forbidden magic that was developed by some of the first magic users. To use this magic, the two users will overload their magical origin to the point their bodies begin to convert their bodies into magic. Upon doing this, their bodies will split apart, then merge into a single form, with the combined features of the two. The created entity is known to have the abilities and memories of the two, but will often have a different personality. This fusion is known permanent and cannot be undone, as well the entity is unable to use the spell again.

Trivia

 * Regardless of the two races used to create the entity, it is always classified as an Isshindōtai.