Demon Seed

A Demon Seed (竜の種 Akuma no Tane) is a plant-like organism that is located inside the body of a Demon, implied to being the source of what causes the process of Demonification.

Description
Like its counterpart in a Dragon Seed, it is implied by Igneel that it causes the transformation of becoming or transforming into a Demon, being located inside the body to which it gathers upon.