Cover is determined as anything that would block a line of sight from one corner of a square to another when firing a ranged attack. If a PC is firing at a monster who has cover against them, they roll with disadvantage. If a monster is firing at a PC with cover the player rolls their defense roll with advantage. In some instances players can gain cover, or soft cover from fight next to walls and using various shields. When using a reach weapon, determine cover as if they attack was being made as a ranged strike.
Cover and Attacks of Opportunity
You can’t perform an attack of opportunity against an opponent who has cover against you.
Cover and Reflex Saves
Cover grants advantage on Reflex saves against attacks that originate or burst out from a point on the other side of the cover from you. Note that spread effects can extend around corners and thus negate this cover bonus (such as Fireball).
Creatures, even your enemies, can provide you with cover against ranged attacks, giving you a +2 bonus to your defense roll except on Reflex Saves.
Big Creatures and Cover
Any creature with a size index larger than 1 determines cover against melee strikes slightly differently than smaller creatures do. Such a creature can choose any square that it occupies to determine if an opponent has cover against its melee strikes. Similarly, when making a melee strike against such a creature, you can pick any of the squares it occupies to determine if it has cover against you.
If you don’t have line of effect to your target he is considered to have total cover from you. You can’t make an attack against a target that has total cover.
Varying Degrees of Cover
In some cases it may make sense to increase the bonuses granted by cover and not claim that a target has total cover. In such cases the bonus for Cover increases to rolling with advantage and getting a +4 to the defense roll.
To determine whether your target has concealment from your ranged attack, use a similar method as determining cover. If any line from this corner to any corner of the target’s square passes through a square or border that provides concealment, the target has concealment.
Melee strikes against opponents completely inside area’s that grant concealment give the character concealment. Strikes made with reach weapons use the same rules as Ranged Strikes.
Concealment Miss Chance
Concealment gives the subject of a successful attack a 20% chance that the attacker missed because of the concealment. If the attacker is a PC and the hit is successful, they must roll a d10 and roll higher than a 2, otherwise they miss. If the PC is the defender and has concealment, they make the same roll except on a 1 or 2 the creature misses.
If you have a line of effect to a target but not line of sight he is considered to have total concealment from you. You can’t attack an opponent that has total concealment, though you can attempt to strike a square you think they are in. A successful attack into a square occupied by an enemy with total concealment has a 50% miss chance instead of the normal 20% You perform an attack of opportunity against an opponent with total concealment, even if you know what square or squares the opponent occupies.
When making a melee strike, you gain advantage on your attack roll if your opponent is threatened by either a friendly character on the opposite side of yours, or if you can draw a line through their square to at least 2 other friendly characters who also threaten them. If an enemy has a PC flanked they roll with disadvantage on their defense roll.Only a creature or character that threatens the defender can help an attacker get a flanking bonus. Creatures with a reach of 0 feet can’t flank an opponent.
A helpless opponent is someone who is bound, sleeping, paralyzed, unconscious, any other status effect that prevents you from taking both actions and reactions.
Ranged and Melee Strikes against helpless defenders.
A helpless character rolls with disadvantage and must use Block as their defense. Their dexterity bonus is treated as if it were -5.
Coup de Grace
As a full-round action, you can use a melee weapon to deliver a Coup de Grace strike against a helpless opponent. You may use a ranged weapon if you are adjacent or in the targets square, but must still make the attack roll. If the target is currently dying you automatically critically hit and must make a fortitude save. The DC of the fortitude save is equal to 10 + ½ the amount of damage dealt. On a failed save the target instantly dies. If the target passes the save you take the total amount of damage dealt - the result of the fortitude save and divide the result by 5 rounded down. The result is the number of Dying levels the target receives, but cannot kill the target. If this would increase them to their maximum level of Dying it is increased to 1 below their maximum instead. Delivering a coup de grace provokes attacks of opportunity from threatening opponents.
You can’t deliver a coup de grace against a creature that is immune to critical hits. You can deliver a coup de grace against a creature with total concealment, but doing this requires two consecutive full-round actions. One to find the creature and the other to perform the Coup de Grace.