Empty

The Empty are those who have, over the course of hundreds if not thousands of years, exhausted their soul's supply of magical energy. This grants them an immunity to higher level sorcery's soul tricks, given that there is no flow to disrupt or intercept. It also renders them immune to most magical sensing, including many forms of targeting. Notably, it leaves them beneath the notice of most demons.
The term Empty originates within the Cotd, the only place in which the phenomena naturally occurs in its purest state, but is relatively widespread in cultures that understand the concept (typically the arcane-focused such as those of The Sunstone Citadels).
Given Cotd immense lifespans and low soul capacities, a significant portion of them have run out of energy in their souls. Becoming Empty is considered a sign of reaching old age within the Darklit Cities.
Empty are relatively rare outside of the Darklit Cities, since an ordinary individual would have to use magic very frequently to do so, even over hundreds of years. Additionally, given the nature of magic use, the individual must then mentally cope with the idea of not being able to use magic anymore after relying on it for so long (with suicide being extremely easy through casting an energy intensive spell).
An odd quirk of Empty is that theoretically, they are capable of absorbing any magical energy directed at them, pouring it into their souls. Ordinarily, magic stored in the soul would naturally resist the foreign source (even if that would kill them, such as a hard light blade to the neck), but without any magical 'pressure', the act becomes possible. Upon doing so, as they now contain the same magical signature (pointer) as the original caster, 'ownership' is now distributed between the two. This in itself leads to all sorts of dangerous possibilities, including allowing the Empty to draw from the magic of the other caster, and said other caster to materialize hard light spikes directly inside the Empty's body to kill them. This concept is unknown to the vast majority of the world, but a small culture has actually made use of this concept in their marriages. More practically, it is the cornerstone of sorcery's flow-disruption techniques.
Under equivalent physical conditions, Empty tire slightly more quickly due to subconscious (typically minor) transmutation of will present in focused tasks.