Spell Sickness

When magic is used, all spellcasters pay a tax of energy- though energy may perhaps be the wrong word. Specifically, when magic is pulled from the fabric of the weave, light is pulled from the environment around them. As a result, this light is known to simply disappear, and it is unknown where it goes.  To overextend one's capabilities with magic causes Spell Sickness, a condition in which a person is temporarily unable to control magic around them. Where they draw their magic from defines how this manifests; for those who draw it from a patron or deity, any interference from the being leads to bursts of wild magic. For those drawing from nature, or their own intellect, attempting magic in this state causes magical energy to be absorbed from the environment, and shockingly similar to vampires, afflicted will attempt to subsist off of this energy and decay the world around them. However, most dangerous are those who use magic innate to themselves; they cannot control when and where magic may manifest from them, or the power of which it manifests- many afflicted with Spell Sickness manifesting in this way die in one way or another caused by their own magic.  It is known, also, that in more powerful spellcasters this condition causes temporary blindness. 
If those afflicted with this are unable to recover, either because they cannot prevent themselves from continually overextending or for reasons yet to be understood, they become mindless creatures that seek only to feast upon the living. It is unknown why they must feast upon the living, those held in captivity and deprived of this slowly fade, eventually crumbling to dust that has been found able to replace any material component of a spell. They may only be killed by completely nonmagical weaponry; any enchantment, even as small as an artificer's use of engineering to enhance a personal weapon or a warlock's blade devoted to their patron renders the weapon unable to kill the afflicted. When successfully killed, the afflicted crumble to the magical dust that can replace all material components. 
It is also known that those afflicted with this stage of Spell Sickness can infect others when they attempt to feast upon them, which has led scholars to posit the theory that they are in some form related to vampires; particularly, a primordial version of vampirism, which by unknown means eventually mutated into the form of vampirism that is known today.

Type

Magical