You normally do 100% damage. Siyalatas gives you a +100% multiplier level 4, so you get double your damage. I'd prefer if it said the full multiplier (ie "200% damage,") but it's not wrong. Further, it's inaccurate to say Hero Souls are additive and Siyalatas is multiplicative. Both HS and Siyalatas stack with themselves additively (each rank of Siy gives 25%, each hero soul gives a 10% bonus.) They stack with your total DPS multiplicatively. So if you have 100 damage, 10 hero souls, and a level 4 (100%) Siy: 100 damage * 2 (+100% HS damage) * 2 (+100% Siy damage) = 400 damage If you were to level up Siy at that point for 5 HS: 100 damage * 1.5 (+50% HS damage) * 2.25 (+125% Siy damage) = 337.5 damage So as you can see, upgrading Siy isn't always going to increase your total DPS. However, since both HS and Siy are multipliers on top of your normal damage, as your HS increases, there will always be a point where leveling Siy will give you a DPS boost.