I never really got into new age or Harry Potter but, in my 40's raising young "curious" teenagers, I knew it was wrong but couldn't tell them "why." So, I began studying scripture and found Deuteronomy 18, which you quoted already. Asking the Holy Spirit to help me explain, this is what came to me................and how I explained it to my children:
There are 3 realms -
The Heavenly - Highest Authority rules over all 3 realms - This is where God Almighty (Father, Son and Holy Spirit) and angels dwell. We are seated in "heavenly places" in this realm in Christ Jesus. The Kingdom of Light.
The Spiritual - Satan, demons, evil spirits, lying spirits, sorcery, witchcraft, new age, etc. The Kingdom of Darkness. Principalities in high places. Has authority over anyone in Physical realm that is NOT in Christ. However, the Kingdom of Darkness cannot do anything without approval from God in the Heavenly realm. Michael the Archangel withstood the prince of Persia in this realm so that an angel (I think Gabriel) could deliver a message to Daniel.
The Physical - Everything and everyone that can be seen, touched, tasted, smelt, and heard - the universe, galaxies, solar systems, planets, stars.
On Earth, the unsaved are just "sitting ducks." Those "in Christ" are seated in the Heavenly realm with Christ and are shining the Light of Jesus Christ to a dark world. We walk on this Earth with power and authority "in Christ" to set captives free, give sight to the blind, bind up the broken hearted, preach the good news of the gospel, go about destroying the works of the enemy. Holy Spirit is in us and with us on the Earth, leading, guiding, counseling, teaching, comforting, empowering us!
I rambled on, hope it makes sense.
All that "spiritual" stuff is real, however, all the "heavenly" stuff is real, too and is the highest authority! Jesus Christ rules over the unseen and the seen. Jesus Christ is Lord of ALL and forever exalted and praised!!