Neo-Demon against Angelic Paragon
The runes in those armors were known as Fiendfire and its original effect was simple yet very powerful. It allowed the carrier to use a very dangerous and powerful type of energy known as fiend fire which was a type of flame that was able to penetrate magic defenses with ease and whose heat was superior to most of the magic flames that existed.
The use of fiend fire had several downsides since its overuse can cause irreparable damage to the life force of an individual and it could corrode their consciousness twisting their personalities.
Zatiel was able to alter the rune, changing its effect, making them consume the life force and energy in the carrier completely before exploding into a ball of fire with the capacity of harming the consciousness of those who touch it.