Necromancy is the act of using magic to bring a person back from the dead, and the only magic that is forbidden in all of Arunia. It is rumored to be the cause of the Old Kingdom's fall. If you go to the Northcliff library in Chapter One and inquire about Necromancy, Rin speculates that it apparently costs one life to bring back another.
In Chapter Two, if you choose to continue a past romance with Zander, he reveals that after Aida's "death," he attempted to learn Necromancy to bring her back.
In Chapter Three, you eventually find out that Aida is technically seen as half dead in terms of magic due to nobles being considered "dead" and her mother being brought to "life" by the Decension ritual. This is why the swords that turn people into Silver Soldiers do not work on Aida; the human part of Aida is trying to transform, but her noble part it trying to stop the transformation.
It is the rarest kind of magic to be born with, and those born with necromancy magic (Soul mages) are killed at birth.