From publisher blurb:
The beast possession fighter is a "minor magecraft" class, which interacts with the magecraft features of the Magus class. These classes are designed to be setting-appropriate to the material the Magus is used for, while allowing players to utilize other classes and not be tied down to a full spellcaster. More will be made available!
The art of self-possession requires a strong mind and body as enduring the transformation can be difficult. These entities evoke the essence of creatures and welcome them inside their body to adopt their traits. This fusion, in its advanced stages, can result in a spectral form: a humanoid rendition of that beast overtaking the form of the user (similar in form to a werewolf, for example).
This magecraft is considered somewhat crude in this form, but is very useful to those looking to apply it to practical situations in the world. Hunters, enforcers, and others find this magecraft particularly useful.
In its most refined form, this magecraft allows the transformation into a powerful legendary beast, mimicking the legends of those most famous of monsters.