Capítulo 225: 4.04.4 There he is again.

"Did you know, the disaster happened due to a mutation?"

"The curse inside the bodies of these Daugrs is a simple one, a normal undead curse that reanimates the body, which was forced under control using the power of a contract with a higher being." 

"Treat it as a sort of restriction on the curse, so that it doesn't leak out or become stronger." 

"While the other part, which is the spirit, magic is used to drive the body and give it a soul. The Victim should be willing in this and agree to turn into a Daugr, only then can a Daugr be born. And all those people you just cut apart were willing in what they had become." 

"Why would someone do that?" Remira couldn't help but ask. Why would someone wish to willingly become a monster? She can see how hollow Farhad feels with his fate; she had read many reports of similar instances where the dead wraiths and spirits that were able to move on, usually cried in torment of their existence.