Retribution Aura is an instant cast aura that surrounds the paladin. If an enemy hits the paladin or a party member that is within 30 yards of the paladin, the attacker will take Holy damage. Like a group version the [Thorns].
Retribution Aura is not affected by +spell damage.
[Aura Mastery] will extend the radius of all the Paladin's auras to 40 yards.
Tips & Tricks
Retribution Aura best suits tanks, as it is a permanent costless source of threat. As it works "on-hit", it is especially suitable for multi-mobs scenarios, as they will hit a lot for (usually) little damage per hit. For a boss fight where the opponent is alone and deal thousands of damage each hit, it is strongly suggested to switch to Devotion/Magic Resistances Aura.
When there's only a single Paladin in a group, this aura is best for a group that can take a few hits. If the Paladin is in a group of cloth wearers, it would probably be better to use the [Devotion Aura] for more armor.
Since there is no resistance to Holy damage, the paladin's enemies will always take full damage (unless they resist it by level).
For tanking Paladins, it is useful as a no-mana always-on form of reactive threat, synergizing well with [Holy Shield] and [Blessing of Sanctuary]. However, it is not recommended for fights where the Paladin needs the full mitigation of his armor, such as boss fights involving massive amounts of damage. Devotion Aura is better suited to these fights, where the idea is to stay alive rather than inflict a minor stream of threat.
+Spell damage gear: as a reactive damage, Retribution Aura do not receive from +spell damage gear, as well as damages are not increased either if the attacker is under Judgement of the Crusader. However, it does benefit the 10% from a possible party Sanctity Aura (the same restrictions apply for Blessing of Sanctuary).
Related Equipment
The Protection-centric Tier 5 Paladin Armor Set, Crystalforge Armor has a 2-piece set bonus that increases all damage done by Retribution Aura by 15.