Simply said - monster X makes attack/cast Y, which the player can not avoid in any way. Whether the player survives Y depends on his EHP.
Should we have unavoidable attacks in Diablo 4 and if yes - which monsters should possess such abilities in order to make the EHP check?
i think point and click abilities are a normal part of rpgs and both players and monsters should be able to use them o:
Unavoidable dmg should be everywhere. Or rather “realistically unavoidable” dmg since of course technically all dmg might be avoidable if you don’t engage an enemy at all.
But in the course of a fight, a lot of the dmg should not be something you can really avoid.
To encourage defensive choices and self sustain choices in builds.
However, that also mean, all these attacks should never ever oneshot anyone but the most extreme glass cannons.
Death should come from “a thousand cuts” and/or getting hit by the attacks which are avoidable, of which there should also be plenty.
Even avoidable attacks should not oneshot most characters though. Slow down dmg taken and healing, for more engaging combat.
But then you’d reduce build diversity since you’d kill glass cannons. It doesn’t make sense at all every monster to possess unavoidable attacks. Not that you can’t make such game of course. It’d just be more limited game, just like the HC mode.
Unavoidable shouldn’t be one-shot or spammable but it should be EHP check feature in the late game, especially boss fight. So if you have crappy HP, lack of sustain and defense, you die no matter how much DPS you have.
Here is the example of the unavoidable attack in the boss fight:
Timestamp - 5:21
Not that this would exist, but what if you had a breastplate of the flame that gave you fire immunity but took 100% increased ice damage. Then certainly you could be getting one shotted.
Hyperbole aside, unavoidable damage should never 100% you. If you are low on HP absolutely. If you wander into an area where mobs are much higher than you(looking at that dungeon that was for 25+ in the 2019 demo when characters were 20), sure.
Avoidable damage should have the potential to one shot everyone, hence it being avoidable. You know like the obviously telegraphed boss big abilities. If you get hit by the Butcher’s (Man Carver’s) charge, and die, that’s all on you. Easily avoidable. I look at thise as being intelligence or paying attention checks.
I have no issues with that. The off screen stuff or far away stuff that you can’t focus on while fighting should just be a % based attack. It won’t ever kill you, but may make it difficult to recover from if you are engaged with too many enemies.
This however reduces build diversity since you directly tell the player the minimum amount of EHP he has to have for late game.
While I could tolerate some bosses in particular Key Dungeons to possess unavoidable attacks, having this everywhere in late game would simply make the game more narrow (just as HC is).
depends i guess
if you just let your armor and resistances unchecked for like 20 levels, it would be a consequence
Yet there’s nothing wrong with that if your tactic is to avoid attacks and focus your build on max DPS.
Forcing the player to play the game in a predetermined way (stack this, stack that etc) limits his freedom regarding build choices (which is what HC already does).
having equipment on level is not a playstyle
i dont know if you know but you cannot choose to wear 14 weapons instead of armor pieces 
I know. We can only hold the 14 weapons in inventory…
Armor pieces however can include offensive properties. Why force the player not stack only these? We already have a mode for that, it’s called Hardcore - permanent death punishing every glass cannon build.
armors still have defensive stats by design which are usually preventing you from dying instantly when being on level
So you want armor default stats to guarantee you are never being one shotted?
tell me an RPG with armors that dont have armor
At the image above we see 1448 Defense.
Let’s take this for the default always included value for level 40 armor. Any armor piece has defense, which the player can not alter. Then no matter if you stack defensive properties or not, you are never going to be one shotted?
probably not by unavoidable instant attacks
but it depends on the balancing team
It would make the game too easy if you are guaranteed to never be one shotted by only having level 40 armor.
One shotting is okay when it comes from avoidable attacks, but when it comes from unavoidable attacks it should be given only to very few bosses in the game imo. This then would force the player to have specific builds with minimum defined EHP for specific Key Dungeons, which would be fine.
If we’re talking about unavoidable attacks, as in attacks you cannot dodge or avoid no matter what you do (you could still defend against them against of course via life shields or temporarily invulnerability), then yeah, I wouldn’t mind them being in Diablo 4. However such attacks shouldn’t be spammable nor too powerful. At most, the unavoidable attacks should be used to inflict a debuff on players, which would then make them even more deadly depending on the situation.
As for one-shots mechanics, if Diablo 4 were an mmo, then I wouldn’t mind. However as it currently stands, players shouldn’t get one shot by attacks unless the players in question are lacking in the defensive stats to survive the attack (for example if players had low life, defenses, resistances, etc.).
Of course that’s not to say that monsters shouldn’t have powerful attacks, as they most certainly should; however such attacks should offer distinguishable signs that helps players to avoid said attacks if necessary (not talking about ground markers ofc). For example, in Diablo 2, Diablo had the red lightning hose attack that can wipe out undergeared/underleveled players. However, Diablo usually pauses briefly before raising both his arms in front of himself when he initiates the attack.
Another example are the dark berserkers in Diablo 3 when they do their massive overhead attack, which can be identitied through several methods, one includes how they swing their mace before the attack.
So in short, yes to unavoidable attacks as long as said attacks aren’t (or can’t be) spammed by the same monster nor is said attack too powerful. And no to one-shot mechanics that one shot players regardless of the player’s defensive stats.
Yes, that could be okay.
I do not see the point in having an unavoidable death mechanic, it’s serves no purpose, has nothing to do with skill and simply inconveniences the players.