Under the spell, the basic damage increases from 6 to 12 hit points and the piercing damage from 3 to 6 hit points. The subtotal of those values is 16, half of which is 8. The Runes only show up once in awhile. Our own axe thrower stepped into a Rune he didn't see. These runes are hidden in the sea.
Runes are kind of like land mines.
An exploding rune causes a damage of 50 hit points to the unit that touches it. That damage is independent from armor and chance. In the heat of battle, especially when fighting on various fronts, you can't always react fast enough and save a unit after the explosion of the first rune before it's already stepped on the next one. The five runes are always arranged in the same pattern. Each rune has a damage power of 50 but can only explode once.