The new defensive buff for Monk comes from The Way of the Monk Quest. To get this buff on your Monk, you need to complete the shrines from the quest.
After the Vocation Adjustments, each shrine from the quest now gives the Monk 2% damage reduction against melee auto attacks, up to a maximum of 20% with all 10 shrines completed. This is in addition to the auto attack damage bonus the quest already gives.
However, it is important to notice that this does not reduce every kind of physical damage, so it will only help against monsters hitting you at melee range, and won't reduce their spells, ranged attacks or other types of physical damage.
The amount of damage it can stop depends on how much melee damage you are receiving.
For example, if one creature's auto attack would normally hit you for 1000 damage, the full buff would reduce it by 200, so you would take around 800 damage instead.
If you are tanking 5 creatures at the same time (on a wall), and each one would hit you for around 1000 melee damage, that would be 5000 total melee damage in that turn. With the full 20% reduction, the buff would stop around 1000 damage in total, so you would take around 4000 damage instead.
It's a very strong defensive buff against melee auto attacks.
If you already completed the quest, you will get it by default without needing to do anything.
