An aspiring Ruler had to defeat the previous Ruler, lest the The Iron Crown of Annuvin scorch the usurper's head unto death. While both known Rulers of Annuvin -- Arawn Death-Lord and Achren -- wielded powers of enchantment, it remains unknown whether a Ruler was required to be a enchanter.
During the climactic events of The High King, Magg sat in the Throne of Annuvin and donned the Iron Crown, assuming rulership without having first defeated Arawn, as Arawn had done to Achren long ago. Magg died shrieking.
It's reasonable to conjecture that, had Magg properly deposed Arawn in some kind of physical or magical struggle, the Crown would have been rightfully his, and so would not have destroyed the former Steward of Mona.