A full flanking bonus of +15.0 to attack negates the equivalent of 15 attribute points invested into dexterity by the target—that is, 5 full levels. Since rogues automatically do critical (backstab) damage when flanking this would be quite a threat, if the number of flanking enemy rogues in the course of an average campaign were greater than. Jan 29, 2016 5E D&D 5E Does flanking grant advantage? Thread starter Awesome Adam. EDIT: The answer I was looking for is found on page 251 of the DMG When a creature and at least one of it's allies are adjacent to an enemy and on opposite sides or corners of the enemy space, they flank the enemy, and each of them has advantage on melee attack rolls.
The Superior Flanking
feat (Dragon # 343, page 93) offers an increased flanking bonus of +6 to attack. It requires Improved Flanking (+4 to attack) as a class ability, though, which limits its accessibility quite a bit. So far I haven't found very many instances of this class feature.
Swashbuckler level 8 (Complete Warrior, page 12)
Rilkan Rogue substitution level 1 (Magic of Incarnum, page 47)
Wolf Totem option of the Barbarian Spiritual Totem ACF (Complete Champion, page 46) grants a functionally identical ability, but without the name