The Warp and souls describes the intimate and perilous bond between every sentient being and the immaterial dimension. In the setting's cosmology, a soul is not merely a metaphor but a real presence, an imprint each living mind casts into the Warp like a shadow thrown by a flame. Emotion, thought, and identity all leave their mark upon the immaterial sea.
This connection is the engine of all psychic phenomena. It is why a psyker can reach into the Warp and draw power out, and why strong emotions felt by the living can stir the tides of the Empyrean into storms. It also means that death is not always an ending. When a creature dies, its soul is released, and where it goes depends on the beliefs, bargains, and predators that shaped its life.
The bond is a source of terrible vulnerability. Daemons hunger for souls, the Dark Gods harvest them, and Slaanesh in particular feasts upon the spirits of the dying, a doom that hangs especially over the Aeldari whose ancient sins gave the Dark Prince its birth. To exist as a thinking being in this galaxy is to carry a light that the darkness of the Warp forever seeks to devour.