Sorry if I wasn't clear, but I did 3 separate Walking Bomb detonations with Dorian on the same Quilback using a save/relaod to keep the Quilback around. Those numbers are meant to represent 3 single targets being damaged, and all of that damage was Spirit Elemental.
I crafted 2 Blades of Tidarion using Bloodstone, so both had 73 base weapon damage. I gave one of the blades a basic Ice Rune, so that one has 85 base weapon damage and 10 AOE Cold Damage.
Okay, I get it. Some maths:
final_damage = (base_damage * rand(0.95 to 1.05) - armor * (1 - armor_penetration)) * (ability_multiplier) * (1 + critical_damage_bonus + flanking_bonus) * (1 + attack_bonus + damage_multiplier + type_bonus) * (1 - magic_resistance)
base_damage = 73
armor = i don't know for Quillback, probably not zero, but ignore for now
ability_multiplier = 900% = 9.0 (Walking Bomb/Concentrated Detonation)
critical_damage_bonus = let's ignore this for now
attack_bonus = 18% = 0.18
everything else is 0
we'll ignore the random stuff too
final_damage = 73 * 9.0 * (1 + 0.18) = 73 * 9.0 * 1.18 = 657
So that would be consistent with doubling the damage, but that's the no rune case! 2x 657 is 1314, right on the money, assuming Quillbacks have no armor.
The rune doesn't seem to make that much difference. It isn't making the base damage 85 (or else the expected damage would be 902, or 1804 doubled). Both the rune and no rune cases have approximately the same damage level, 1350ish vs 1317 respectively.
It's broken, but it may be broken in many different ways, not just by adding a rune.