Necromancy is a yet-to-be-implemented skillset that would allow one to create new vessels, with varying degrees of quality.
Per default, a player inhabits a vessel on the start of campaigns. These vessels, however, are only of common or 'white' quality. Using race-specific body parts harvested from cemetaries (grave digging skill), coupled with alchemist's solution (alchemy skill), one is able to craft new bodies.
Necromancy allows one to 'upgrade' his/her vessel based on attributes. These attributes will be changed based on primarily three things: the archetype vessel made, the quality of resources used and and optional materials added during crafting.
Firstly, Attribute levels are specific per archetype, with one being the main attribute. The knight is mainly strength based, while the ranger is mainly dexterity based, the confessor being intellect based, etc.
Secondly, the quality of the resources used will allow more and varying experimentation to be done on the vessel and it's parts. This experimentation will increase the base attributes of this vessel based on the archetype you wish to make.
Lastly, optional materials, which are to be found as critical hit rewards from gravedigging, can be used to enhance the vessel even further in certain aspects, tailoring it to the needs of a specific job a player does, like crafting, fighting, harvesting, etc.