Steeples

'Steeple' is a pet name for a certain type of natural rock formation. One which seems to only form deep underground and is made of near indestructible, mana-infused rock.
Steeples react faintly when magic is used near it, which has led researchers to believe that they channel magical energies deep from within the planet. When magic is cast directly at them, however, they react a little more violently.
When a magical effect touches a steeple, it causes an intense concussive force to radiate from the point of contact, a force which is greater and affects a larger area the stronger the spell that touches it.
Further investigation unveiled that despite their magical nature, steeples are a magic nullifier and if a magical item comes into contact with them, the explosion dispels the enchantments cast on it. So far, there has not been an item discovered that is immune to this, though researchers aren't enthusiastic to test the explosive properties of powerful artifacts.