Maxperson
Morkus from Orkus
For the people who like to play the way I am talking about, it is player skill. I'll give an example.how is could that be the failure of player skill - a 0HP death mechanic is nothing that happens in the narrative, its a pure mechanical effect. If you try to telegraph it in the narrative "this monster seems really deadly to you" or something like that, most players will think "oh they probably make tons of damage". I cannot fathom how someone would think it is a players failure to not correctly deduct from the narrative that this monster deactives a full component of game system.
The worst part of the mechanic that we haven't even talked about (correct me if I'm wrong): There is also nothing the players can do about it afterwards, they just have to accept it. I would rather implement something like "because of this masterful necrotic evil spell, you fail your death saving throws at 14 or lower" (like in Tomb of Annihilation) or something like "because of the incredible deadly wound this attack inflicted on you, you only need 2 failed death saving throws to die" because than you have the surprise, but still can react to it.
A mechanic that can't be anticipated, so the players can't make informed decisions to prepare, and that can't be reacted afterwards to diminish the damage, so the players informed decisions afterward are also limited - thats a bad mechanic to me for an TTRPG. Because roleplay is about making informed decisions, even in combat. Sure surprises are exciting, but they must leave the player with a new obstacle, a new problem to overcome, not just with an "oops youre dead". As @James Gasik said correctly IMO, mechanics like these reduce the game to a game of luck.
And OSR games or older D&D editions had a lot more of insta-death traps and abilites - so players knew they existed and anticipated them. 5e normally don't have them, so players have a much harder time to anticipate them.
Suppose the party enters a new city and go to the local bar. While at the bar they overhear some locals talking about The Ruins of Doorkey. They decide that it sounds interesting and want to explore them.
The first thing they do is ask around about the ruins. They find an NPC who tells them that the deadly Morkfromork dwells there and nobody returns from the ruins.
Now the party is concerned, so they want to research what a Morkfromork is and can do. So they go to a library and after a time they read a passage that tells of a deadly energy that transfers from a Morkfromork if it should strike solidly with all six claws of its right hand. If that happens, death is instant.
Since only the final hit that takes you to 0 is a solid, damaging hit, they know that they need to avoid dropping to 0 against it.
Perhaps they try to find a sage that knows more to see if there is a way to defend against the attack.
That's player skill in action.
Or they don't do any of that and just go straight to the ruins from the tavern and it's all a surprise. That is a failure of player skill.