Flanking

When multiple creatures within 5 feet of another creature engage it in combat, that creature may become flanked. This variant rule offers mechanical boons to flanking separate from simple advantage.

A creature who is one-quarter flanked suffers a -2 penalty to AC and Dexterity saving throws against sources within 5 feet of it. A creature is one-quarter flanked if it is at least one-fourth surrounded, either to a side, above, or below.

A creature who is half flanked suffers a -3 penalty to AC and Dexterity saving throws against sources within 5 feet of it. A creature is half flanked if it is at least half surrounded, either to a side, above, or below.

A creature who is three-quarters flanked suffers a -5 penalty to AC and Dexterity saving throws against sources within 5 feet of it. A creature is three-quarters flanked if it is at least three-quarters surrounded, either to a side, above, or below.

Alternatively, you can have players add a +2, +3, or +5 bonus to their attack rolls against creatures who are one-quarter, half, or three-quarters flanked, respectively, by them. Additionally, if you use the Saving Throws Checks variant, you can instead have them add a +2, +3, or +5 bonus to their Dexterity saving throw checks against creatures who are one-quarter, half, or three-quarters flanked, respectively, by them.

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.

Flanking on Squares. On a square grid, a Small or Medium creature takes up one square, and has eight squares surrounding it within 5 feet. When two or three of those squares are occupied by hostile creatures, that creature is considered one-quarter flanked by those creatures. When four or five of those squares are occupied by hostile creatures, that creature is considered half flanked by those creatures. If six or more of those squares are occupied by hostile creatures, that creature is instead considered three-quarters flanked.

Flanking on Hexes. On a hex grid, a Small or Medium creature takes up one hex, and has six hexes surrounding it within 5 feet. When two of those squares are occupied by hostile creatures, that creature is considered one-quarter flanked by those creatures. When three or four of those squares are occupied by hostile creatures, that creature is considered half flanked by those creatures. If five or six of those hexes are occupied by hostile creatures, that creature is instead considered three-quarters flanked.

Alternatively, you can run the simpler ruleset for flanking found in the DMG, that offers advantage instead of flat bonuses. That ruleset works alongside Advanced Advantage as well.

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.