Grief Seeds are a product that form from the defeat of a witch, in this world a witch is a creature sprung from a magical girl's death and is a personification of all of the grief, wrath, hatred, anxiety, etc. that they have accumulated throughout their life. Grief seeds are also a magical girl's reward for defeating a witch and drains all grief that have accumulated in their Soul gem, a device that their soul's are kept safe in.
Puella Magi Madoka Magica
Puella Magi Madoka Magica written by Gen Urobuchi
Puella Magi Madoka Magica, “Grief Seed,” The Museum of Fictional Literary Artifacts, accessed January 25, 2022, https://mfla.omeka.net/items/show/96.