Just an idea but the revive book made them grow horns, like Schlatt horns grew a bit larger because of use with the revive book, but it was so little that they only got a bit bigger.
Dreams horn also only started to grow while experimenting with the revive book.
Maybe it’s some kind of curse? A tell of who’s or acting dark magic? Maybes it’s the effects and strain of using such a powerful spell that it forces literal bone to move up to the skull.
Your head is your central magic after all : )
Also dream gets migraines whenever he using the book