What's that marking on his horn?
Amorphic Crystal. In my fanfic changelings from Greywilds are willingly allowing a small crystal like creature to live in them. Process is simply good for both parties as Changeling gets boost in his magic or other attribute and the creature got a host to live in and feed on excess of its magic. Each changeling with such creature in it gets a visible crystal or few on horn or on shell. Crystals can be random shape and colors