Diablo 4 players are already breaking the new Horadric gear system
Horadric affixes appear to stack additively, letting some builds ignore incoming damage and accumulate over 100% damage reduction.
Diablo 4 Season 15 currently has a Damage Reduction problem, but it seems like the Season 16 PTR is the exact opposite. Although it depends on how you look at it...
Diablo 4 players are already finding some ridiculous combinations with the new Horadric items system on the Season 16 PTR.
It seems like some builds can stack enough Damage Reduction to go beyond 100%, pretty much preventing incoming attacks from dealing any damage. Yes, you are reading this right, and no, this should not be possible in theory.
The issue appears to come from multiple Damage Reduction bonuses stacking additively instead of multiplicatively.
With the right combination of Talisman set bonuses and the new Horadric affixes, you can push your Damage Reduction past the point where you should be taking damage at all.
How to Reach +100% Damage Reduction
In Season 16, you can drop Legendary items that come with one additional Horadric affix that is connected to your Talisman sets. Most of these affixes are Damage buffs, but you can also get things such as Damage Reduction bonuses.
The affixes you can get on the Horadric items are basically the same ones you get on your Set Charms.
From what we could see so far, it seems to work mostly on Barbarians, but it should also be possible to reach on other classes.
A lot of sets can provide Damage Reduction, such as 
Berserker's Crucible. Sometimes the effects are conditional, so you need to meet the requirements before the bonus gets added.
The problem is that these bonuses currently appear to stack additively.
Basically, instead of having each new source reduce the remaining damage by another percentage, you can combine the values until they reach +100%.
We found multiple posts from content creators like Cliptis or Wudijo that have shared examples of Barbarian builds reaching this +100% Damage Reduction on the PTR.
At that point, all the incoming damage is just negated, and you look completely invulnerable.
wudijo@wudijo𝕏"Just tank it", Barb edition. On the S16 PTR you can just stack 100% damage reduction and nothing can hurt you. Par for the course, I guess 😂 #DiabloIV
Is This How Damage Reduction Is Supposed to Work?
No, probably not. Getting your incoming damage reduced is supposed to just make you more tanky, not completely invulnerable to everything. It would be surprising if this was intended by Blizzard.
Let's go over a small example:
How affixes normally stack
If two items give you x50% Damage Reduction bonuses each, you would still take around 25% of the incoming damage, because they stack "multiplicatively"
On Horadric items
They add to each other. If you get two items that grant +50% Damage reduction, you get 50% + 50% = 100% damage reduction, and that negates all the incoming damage
It also makes us question how much you would need to invest in defence.
Right now, if you want a really tanky build, such as the 
Melted Heart of Selig immortal Barbarian, you often need to sacrifice other things. But here, if all you need are a couple of Horadric items with the right affixes, it could free up skill points, Paragon nodes, and other gear slots for additional damage.
So you would be pretty much immortal, and at the same time, do even more crazy damage.
From what we could see, most seem to agree that there should still be a trade-off between survivability and damage. Otherwise, it will just be unfair during Pit/Tower pushing.
Will Blizzard Fix This Before Season 16?
We would be surprised if this stays unchanged until launch.
The PTR exists to test new systems and identify balance problems, and allowing players to become invulnerable this way would have too big an impact on endgame.
Blizzard has different ways they could make this better:
Make the bonuses on Horadric items stack multiplicatively like they always do
Add some sort of penalty in return
Limit each type of charm affix to only one item (so for example, if you have Damage reduction on your Chest, it will be greyed out if it is on any other item)
Either way, do not take it as a confirmed Season 16 feature. The chances of you being able to accumulate over 100 Damage Reduction on live is rather low.
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