The Necris was the starter motor designed for Hervoken ships known as "Sky-hearts". It had the appearance of a book and allowed energy to channel around the ship and into the Hervoken themselves; powered by their unique form of magic which could also be used to recall the book if lost.
The abilities of the Necris, like all of Hervoken magic such as the use of agents, could be weakened temporarily through the use of salt or, more effectively, iron. (PROSE: Forever Autumn)