-If a human has a brand, it is an indicator that they are dead. Any human soul not wearing the highly conservative clothing given to them by a Chiranoble/Bezzafrayl is likely to be corrupted by the black sun.
-Even though souls bleed and feel pain, the blood itself is not of actual flesh qualities given how much a soul can bleed without "dying". It is difficult to actually "kill" a soul through means that killed the original human such as stabbing, cutting, etc. Other than this, a soul "heals" much the same way an actual human body would (an amputated limb will not return, wounds heal gradually over time, etc.).
-A demonstrated way to remove a soul from Mionfaer is to "return [it] to the mortal cycle."
-As a soul is exposed continuously to the black sun, their form will begin to corrupt and warp. The process itself is said to be excruciating to the point of madness.
-Some of the accompanying ailments of corruption are blackened flesh, difficulty breathing and intense weakness. Some of the more developed ailments of it would be that the respective soul's skin will crack, bleed and continue to change into something else instead of heal.
-It takes at most two days (of constant exposure) for the corruption to fully consume someone or that respective part of them.
-A way to cure a Nihilfaer is to purify them with the fire of a Chiranoble.
-Another way to deal with the process is to physically remove the infected area from the rest of the body.
-Nihilfaer are implied to be violent and otherwise a threat to other's safety.
That creature in chapter 3...
-In chapter 3, Sidsael describes a creature that is neither Familiar nor Nihilfaer.
-He, as well as the Unmarked Magician, later encounter it, engage it in combat, and it undergoes a strange transformation...
Extraction/Dead Familiar? (Chapter 4)
-In chapter 4, Gerif is shown to have the ability to summon a black and red blade. He says that he can call the blade due to someone promising to help him "use magic without Astervotum". However, as a result he cannot summon anything BUT that blade. He describes the process as "pulling something black out, doing something to it and putting it back in."
-Gerif suggests that the blade is his dead Familiar.