For simplicity's sake, supposing that Ember Spirit has a Quelling Blade which deals 50% damage to non hero units, and Ember spirit has 50% cleave from whatever source, and that hypothetically Sleight of Fist does a static damage value of 100 to whatever unit it hits and this 100 damage is never reducted, if I hit a creep unit that stands beside a hero, how much cleave damage will the hero take?
A: 50 damage, meaning no Quelling Blade bonus damage added to cleave, i.e. creep is hit for 150 damage, but only 100 damage is accounted for cleaving purposes.
B: 75 damage, as the creep is hit for 150 damage due to the hipothetically 50% bonus damage of quelling blade.
C: None of the above.
Best Answer
The answer is B or C depending on what you mean with 100 damage :
If you're supposed to deal 100 damage to the creep you're about to hit for whatever reason, then Sleight of Fist doesn't matter at all and the calculation is : 100x(cleave%)x(1+quellingBladeBonus%) = 100x(50%)x(150%) = 75 damage. The answer is B
If you're supposed to deal 100 damage to heroes, then the calculation is tricky and requires to know the level of Sleight of Fist you have because it deals both bonus damage to heroes AND 50% of normal damage to creeps. The answer is C
I guess your question was regarding the interaction of Quelling Blade and Cleave, and Sleight of Fist is not really necessary here as it's just a source of confusion. The answer is : Cleave and Quelling blade do stack, if you are late in the game and you happen to have cleave% x (1+quellingBladeBonus%) > 1 (disregarding armor) you're better off hitting creeps than heroes if you have the chance to.
Note that it's not reliable with Sleight of Fist as you don't have a lot of control over the angle you hit the creeps from.