A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.
Here’s how to determine a character’s total for a passive check:
10 + all modifiers that normally apply to the check
If the character has advantage on the check, add 5. For disadvantage, subtract 5. The game refers to a passive check total as a score.
For example, if a 1st-level character has a Wisdom of 15 and proficiency in Perception, he or she has a passive Wisdom (Perception) score of 14.
-PHB, “Using Ability Scores” (Ch. 7)
Game masters in Chromatia will and often do make use of passive skills in regular gameplay, but it should be known that Chromatia doesn’t usually permit using passive skills as a “skill floor” outside of very specific circumstances.
Consider the context of “monster hiding in room”. You have a passive perception of 16 and ask if you can make a perception check as you enter the room to search for anything hiding in wait (unbeknownst to you, with a DC of 15 to find). You roll a 14, which is lower than your passive perception – even still, the result of your check is 14, a result of your active search. You did not find the monster in waiting when you searched the room. Unless you linger in the room or meet some other criteria for your passive skill to apply in this context, your passive skill does not replace the active skill.
After that, it’s at the game master’s discretion; for example, the game master may determine in the event of a surprise attack that you are not surprised. Despite not finding the monster initially, the GM determines your passive perception is enough to notice it come out of hiding in the moment before the attack. Sometimes, a monster is hidden so well that the DC for finding it with a passive skill is higher than finding it normally, to represent it having advantage on the hiding check.
Whatever the case, the GM has final say on whether to permit your passive score to apply or not.
