Soul-Weaving

Soul-weaving is an outlawed form of magic, similar to blood magic. Soul-weaving is a magic that utilizes the caster's soul, along with souls they are able to obtain for a certain amount of time. The spells of soul casting are rumored to have been passed down in hushed circles, and are still alive today. However, this ancient secret is not easily administered. The danger of soul-weaving is that the magic can easily change based on the soul of the user, creating unpredictable and fluctuating levels of power.
For those fortunate enough to persuade a Soul-Weaver to teach them, they have to learn the magic individually, and not every Soul-Weaver is equipped with all spells. For classes with the spell casting class feature, the ability score used to cast soul magic is their spell casting class feature ability score. For martial classes, use constitution.
Soul-Weaving Spells do not work the way regular spells do, with the level of the spell correlating with the level of the spell slot. The spells that are woven in the soul can only be handled by weavers of a certain level. Characters can learn soul-weaving spells at any level, but trying to cast spells that are too strong for the soul can be catastrophic.
Soul-weaving slots are attributed by temporarily absorbing souls of fallen enemies. Soul-Weavers, after killing, can choose to absorb the soul, giving them one soul-weaving slot. You can hold a number of Soul Slots equal to your proficiency modifier. These slots are expended when a Soul-Weave spell is cast, or when the holder of the slots falls unconscious. These slots become higher level slots after your proficiency bonus increases by leveling up, (i.e 5th level, 9th level, 13th level, and 17th level.)
However, souls captured in this way and then expended are not lost forever, and after the soul is used within a soul-weave casting, the soul then acts as it had just died. Souls are then sent to the afterlife and rejuvenation or reviving spells work as normal.