If, for example, someone casts Fog Cloud, next to the target A and target B when they're fighting each other. If target B would run away from the fight he would cause an Opportunity Attack but when the area is Heavily obstructed (by the Fog Cloud spell) all the targets are counted as Blinded.
- A blinded creature can’t see and automatically fails any ability check that requires sight.
- Attack rolls against the creature have advantage, and the creature’s Attack rolls have disadvantage
In a fight, everyone is constantly watching for a chance to strike an enemy who is fleeing or passing by. Such a strike is called an opportunity attack. You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
If target B tries to leave target A's reach can target A use his Perception to beat target B's stealth? Or is target B is not detected, since target B cannot be seen? Or is target B forced to use Disengage even though he can't be seen?