Lily’s entering the Precreated World poses a threat to that universe in its entirety. Material from one universe has very different properties if introduced into another universe. In this alien world the atoms she is made of contain so much energy that it could cause something like a big bang in reverse if handled carelessly. Just one hair from her head can now fuel the stars of thousands of galaxies as long as those galaxies belong to the Precreated World.
If a foreign object is introduced into a living organism the organism will try to destroy it, push it out or build up protective calcium around it.