Spirits are one of the two primary types of manifestation— the other being mythics. As a primary manifestation, spirits are autonomous creatures that are entirely composed of magical energy. They have no physical shape, and can only be seen by people who have a connection to magic in themselves. This makes them difficult to manage, since they can interact with nonmagical humans without any fear of retaliation— but many spirits are content to live in obscurity, away from witches who might cause them harm.
Like mythics, spirits are born from magical fields and humanity's collective unconscious. They tend to take less elaborate shapes, preferring the forms of simple animals or less-than-common myths. Additionally, all spirits share one ability: The ability to possess living things. In extreme cases, possession can transform the victim into a copy of the spirit, a process known as zoanthropic possession.
While attached to a source of magical energy, spirits are essentially unkillable.
Possession is dangerous for both the spirit and the victim, but humans have devised a series of countermeasures to make it safe. First, a "palm weaver" is stitched into the spirit weaver's skin on their left hand (usually in the form of a glove or a ring). Then, through a weaving ritual, the spirit is compelled to possess the palm weaver, locking it into place without sacrificing the weaver's body. By manifesting their spirit, a spirit weaver is able to use its abilities as their own— a powerful tool in the hands of a skilled magic user.
Spirits who have been woven are not always docile. They receive the same benefits that they would otherwise get from possession: A source of magic and greater control over their surroundings. This can compel some spirits to behave, but others are not so happy to be taking orders from a human. For this reason, it's important for spirit weavers to befriend (or placate) their spirits in advance, lest it attacks the weaver in an attempt to escape captivity.
Magic users of all types can become spirit weavers, but some benefit from it more than others. In particular, it's possible for ordinary humans to gain access to magic by having a witch weave a spirit into them— one of many ways for a normal person to be introduced to the world of magic.