Cover, Elevation,
and Flanking
Cover
Objects, foliage, creatures, and other obstacles can provide cover during combat, making a creature more difficult to harm. Cover is directional; a creature only benefits from it when the source of an effect originates on the opposite side of the cover.
There are four degrees of cover: three partial and one full. If a creature is behind multiple sources of cover, only the most protective degree of cover applies.
A creature that’s lightly covered has a +2 bonus to AC and Reflex saving throws. A creature has light cover if an obstacle blocks at least one-fourth of its body, such as a thin pillar or a creature of one size category smaller.
A creature that’s moderately covered has a +3 bonus to AC and Reflex saving throws. A creature has moderate cover if an obstacle blocks at least half of its body, such as a low wall or a creature of the same size category.
A creature that’s heavily covered has a +5 bonus to AC and Reflex saving throws. A creature has heavy cover if an obstacle blocks at least three-quarters of its body, such as a large rock or a creature of one size category larger.
A creature that’s fully covered can't be targeted directly, although such a creature may by included in an area of effect. A creature has full cover if it is completely concealed or otherwise protected by an obstacle, such as a wall or impenetrable glass.
Creatures Providing Cover
Sometimes, creatures in close proximity create a veritable mess of bodies, potentially causing attacks targeting one to hit another. Using optional addition, if an attack would miss a creature by an amount less than or equal to the bonus to AC granted by the cover of another creature, and the attack would hit that other creature, the GM might consider having that other creature take the attack's damage.
Elevation
A creature making ranged attacks might choose to do so from an elevated position to gain a benefit against its target. Doing so grants them a bonus called dominance, which is measured in three degrees.
A creature is lightly dominant over another creature if it is at least 10 feet above that creature vertically, and at least 10 feet away from that creature horizontally. While a creature has light dominance over another, it has a +2 bonus to ranged attack rolls against the dominated creature, and the dominated creature suffers a -2 penalty to Reflex saving throws against effects the creature dominating it controls.
A creature is moderately dominant over another creature if it is at least 20 feet above that creature vertically, and at least 20 feet away from that creature horizontally. While a creature has moderate dominance over another, it has a +3 bonus to ranged attack rolls against the dominated creature, and the dominated creature suffers a -3 penalty to Reflex saving throws against effects the creature dominating it controls.
A creature is heavily dominant over another creature if it is at least 30 feet above that creature vertically, and at least 30 feet away from that creature horizontally. While a creature has heavy dominance over another, it has a +5 bonus to ranged attack rolls against the dominated creature, and the dominated creature suffers a -5 penalty to Reflex saving throws against effects the creature dominating it controls.
A creature can only benefit from one level of dominance at a time for each creature it would be dominating, but it can have different levels of dominance for different creatures. For instance, if a creature is 30 feet above another creature both vertically and horizontally, it is heavily dominating that creature. If it is 20 feet above another creature vertically and 15 feet horizontally, it is only lightly dominating the third creature instead of heavily dominating, since both values exceed 10 feet, but not 20.
Elevation and Cover
if a creature is both dominated by another creature, and it has partial cover from that creature, you don’t add the bonus and penalty together. Instead, you reduce a creature’s dominance by one for each level of cover it has. For instance, if a creature is moderately dominated by a creature, but it is also lightly covered from that creature, it is instead just lightly dominated. If the creature that is dominating it has a feature that allows it to reduce or ignore the benefit of cover, such as a Sharpshooter Technique, however, the creature is still moderately dominated since the creature dominating it isn’t affected by the cover.
Flanking
When two or more creatures within 5 feet of another creature engage it in combat, and at least one of those creatures is on the opposite side from another, that creature may become flanked. There are three degrees of flanking.
A creature who is lightly flanked suffers a -2 penalty to AC and Reflex saving throws against sources within 5 feet of it. A creature is lightly flanked if it is at least one-fourth surrounded.
A creature who is moderately flanked suffers a -3 penalty to AC and Reflex saving throws against sources within 5 feet of it. A creature is moderately flanked if it is at least half surrounded.
A creature who is heavily flanked suffers a -5 penalty to AC and Reflex saving throws against sources within 5 feet of it. A creature is heavily flanked if it is at least three-quarters surrounded.
Alternatively, you can have players add a +2, +3, or +5 bonus to their attack rolls against creatures who are lightly, moderately, or heavily flanked, respectively, by them.
The amount of creatures it takes to flank another varies based on the involved creatures’ sizes. A Medium creature can easily be lightly flanked by two Medium creatures, provided they are on opposite sides. A Large creature could not be, however, since the Large creature takes up more space, and two creatures could not surround one-fourth of it. Conversely, two Large creatures flanking a Medium creature would be able to surround half of it, making it moderately flanked.
A creature who has a reach of greater than 5 feet gains the benefit of flanking against a creature while within melee range but does not contribute towards flanking unless they are within 5 feet of the target.
A creature must be conscious to contribute to flanking, and it must be able to see the flanked creature in order to contribute towards flanking or benefit from it.
A creature who is flanked cannot participate in flanking another creature, though they can benefit from that creature otherwise being flanked. Many constructs, such as golems, vehicles, and turrets, cannot be flanked at all.
Standing From Prone While Flanked
When a creature attempts to stand from prone while flanked, it can choose to make a Strength (Athletics) or Dexterity (Acrobatics) check (DC = 10 + its penalty from flanking). On a success, the creature stands and can move to an unoccupied space within 5 feet of it without provoking opportunity attacks. On a failure, the creature remains prone and loses the movement it would have used to stand.
Simplified Flanking
Instead of the above rules, you can instead use a simplified version of flanking that grants advantage on attack rolls against the flanked creature. You can also include rerolls for moderately and heavily flanked; allowing rerolling one of the dice once for moderately and rerolling each of the dice once for heavily.