Sometimes you want to do something and rolling 1 dice does not accurately portray the complexity of the action. This is where Complex Skill Checks come in. How they worked:
When the DM or an action calls for a Complex Skill Check you check the skill it is asking for, you remove your stat modifier from your skill bonus and you roll the check without the stat modifier bonus. But for every bonus you do have you get to roll 1 extra dice. So if you got an Int of 20, then in that complex Int-based skill check you would roll 5 dice and check vs. the DC given to see how many passed. In most cases, the DC is 10 + CR of event testing against.
If you roll a natural 20 when making complex skill check, you get to roll that dice again and count the new roll as well!
Opposed Complex Skill Check
When testing against someone else you both get to roll vs. a DC and then depending on the action either the one that gets the most passes wins, or each pass cancels the other one out and you need to get so many passes to complete the action against the other person.
Let's take an arm wrestling match
one wrestler(A) has 20 str other has (B) 18
So in round one, each would make a str check, A rolls 5d20 against a DC of 10+ opponent str mod +4=14, and B rolls 4d20 vs DC 15. Neither adds their str mod to the rolls.
When testing vs. another person in most cases it is DC 10 + the stat or DC 10 vs. skill you rolling against, so the one that has a higher rank and/or higher stat has a better chance of winning, which is how it should be.
Complex Skill checks are used in two ways by the DM. To reflect the action is not just a quick 1 roll action that you overcome with ease, or to build suspense as part of the story, rushing to get so many passes before doom falls on the party.
