From Dark and Darker Wiki
< True mag dmg and ray of darkness and flame walker?
Ray of Darkness: Adds at a 1:1 with the 12 per second, so if you have +3 true magic damage, Ray of Darkness will deal 15 damage per second + whatever scaling from Magic Power Bonus on the 12 base damage. For example, if you have 20% MP Bonus & +3 true magic damage, it would deal 1.2 * 12 = 14.4 + 3 = 17.4 magic damage per second.
Flame Walker: Adds at a 1:1 with the 25 per second, so if you have +3 true magic damage, Flame Walker will deal 28 per second (split into 0.2s ticks). So 28 / 5 = 5.6 per tick + whatever scaling from Magic Power Bonus on the 25 base damage. For example if you have 20% MP Bonus & +3 true magic damage, it would deal 1.2 * 25 = 30 + 3 = 33 / 5 = 6.6 damage per tick or 33 per second.