Heritrix

Heritrixes (hare-uh-trixes): Heritrixes are immaterial beings, sometimes confused for ghosts, demons, or other sorts of supernatural spirits, who enter into contracts with physical hosts. In exchange for their expertise and magical power, Heritrixes are allowed to take control of the host’s body for an agreed upon period of time, giving them a way to experience the material world. I’d say that I wouldn’t sign an agreement like that, but who’s to know?

– The Almanac of the Heartland Rider

Heritrixes (/ˈhɛrəˌtrɪksɪz/) are a type of creature in Sangfielle with no physical form who create contracts with a host body that they then possess for a set period of time. The relationship between a heritrix and their host is consensual.

Heritrixes come into being in concentrated pockets and then spread out throughout the world. While not in a host body a heritrix is seen as a kind of white or gold mist. Many heritrixes are quite old, being functionally immortal and passing between host bodies when their contracted time passes. As such, heritrixes often use many names, though between other heritrixes the name they use is often more related to the time and place they came into being (this is described out of character as something akin to the metadata in a photo).

While in a host body a heritrix is able to communicate with their host's consciousness, and can also partially revert to their true form. This is often seen with Es when she uses her true form for combat.

This section contains spoilers for Sangfielle 31: Hark! The Citadel Beneath Pt. 4.
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The Glim Macula in Sapodilla were kidnapping heritrixes and forcefully separating them from their hosts to be used as materials for various experiments and objects with 'magical effects'. They were freed by Es as the Blackwick Group explored the Glim Macula's citadel.