You compute two ratios first:

Attacker Cunning / Attacker Level+10
Defender Vitality / Attacker Level+10 (yes, Attacker Level+10 here too)

So, let’s say a Level 5 demon with 20 Cunning is trying to land a negative effect on a Level 3 demon with 10 Vitality. These ratios turn out to be:

20 Cunning / 15 = 1.33
10 Vitality / 15 = 0.67

You take these ratios, and filter them through a chart that looks like this:

Ratio -> %
0 -> 75%
1 -> 100%
2 -> 125%
3 -> 150%
etc.

For 1.33, we get a value of 108%. For 0.67, we get a value of 92%.

Finally, divide the first ratio’s result by the second. In this case, 108% / 92% = 117%. This is the percentage of the tooltip listed chance you have to land the negative effect on your target.

Weakness, Resistance, and Immunity also apply: Weakness and Resistance give large bonuses/penalties to landing associated effects, and of course, Immunity prevents it outright. Heroes also have a large amount of bonus resistance to all negative effects.

I’m trying to get better about explaining this stuff in game… but I’m not sure how I could effectively in a bite-sized fashion. Maybe it doesn’t need to be bite-sized and I should put these explanations in the back of the manual for any interested parties to read?