In Path of Exile, defensive items are essential for surviving in the game's harsh environment. The Fourth Vow is one such item, introduced in patch 3.21. Despite being underused, it is an extremely powerful defensive item that provides excellent protection to your character Build. In this guide, we will discuss how to use the Fourth Vow effectively.

What is the Fourth Vow?
The Fourth Vow is a devout chainmail that provides armor that applies to chaos damage taken from hits. This means that in addition to mitigating physical, elemental, and other damage types, your armor also reduces the chaos damage you take. It does not make you immune to chaos damage but helps to minimize its effects, especially in conjunction with high chaos resistance.

How to use the Fourth Vow effectively?
To make the most of the Fourth Vow, you will need to invest in chaos resistance. By default, characters can have up to 75% chaos resistance, but with the Fourth Vow, you should aim to push it even further. A Javaqua version of the unique Timeless Jewel, Glorious Vanity, can grant you Divine Flesh, increasing your chaos resistance cap to 80. Additionally, you can use a Crucible Shield, which, when crafted with poe currency correctly, can take your chaos resistance to 87 or even 89.

To further increase your protection, you can use the "damage taken as x" mechanics. The Fourth Vow and other sources can convert some of the physical damage you take into chaos damage, reducing the total amount of damage you suffer. A mastery that provides 10% of physical damage taken as chaos damage is an excellent option. Additionally, a helmet that drops in Delve can have 10% of this stat as an explicit mod, which can be further augmented with Eldritch Crafting.

The interaction between armor and chaos damage is crucial when using the Fourth Vow. Armor mitigates each damage type separately. If you have both physical and chaos damage incoming, your armor will reduce each type of damage independently. For example, if you take 10,000 points of physical damage, and you have 50,000 armor on your character, you will mitigate half of that damage. If 50% of that incoming damage is converted into chaos, you will take 5,000 physical damage and 5,000 chaos damage. However, if you have 80% chaos resistance, you will only take 1,000 points of chaos damage. Your armor will then mitigate 90% of the remaining damage, which in this case is 4,000 chaos damage, resulting in only 100 points of hit-point loss.

Conclusion
The Fourth Vow is an excellent defensive item in Path of Exile. By investing in chaos resistance and using the "damage taken as x" mechanics, you can significantly reduce the total amount of damage you take. Understanding the interaction between armor and chaos damage is crucial when using the Fourth Vow, and by doing so, you can maximize its defensive potential.