True. I admit this particular group was built for a Justice League type of adventure: defeat the Demon Lords of The Abyss. If that isn't a superhero level fantasy adventure, I don't know what is.
Ignoring? More like already aware of all those options. I understand and have used many times the resource depletion tactic to make an encounter more difficult. I don't need advice, not even a little bit. I've done everything mentioned and more many, many times before. This isn't some neophyte asking for advice. This is someone discussing the math of the game. I gather not many people spend time concerned with the math of the game or they would see the problems I'm talking about with expertise with Stealth and Perception against the environment using the Passive Perception rule, certain spells, certain feats, and some of the monster design in the MM. Some things are better than others and it is clear when you look at the game math. Something I've done in every edition of D&D.
The math of skills, spells, feats, and monster design is not your problem. That's why nobody is talking about it.
It is beyond obvious that your decision to
ignore the math that the game is designed around is what is breaking your game.
You rolled 4d6 drop the lowest seven times.
You handed out triple legendary artifact magic items to everybody.
You allow everybody to concentrate on extra buff spells.
You allow Unearthed Arcana material that is playtest quality at best.
You created another -5/+10 feat for sword-and-boarders to use.
You allow players to hold three uses of Inspiration at a time.
Those were all
your decisions. Those decisions were not the PHB's, not the DMG's, not the MM's, not UA's, not OotA's, not Jeremy Crawford's, not Gary Gygax's, not flamestrike's, not Hemlock's, not mine, not your PCs'. It was
your decision to permit every single one of those things (and whichever ones you've not told us about yet). Your lack of acceptance of any responsibility for the role that you have played in your current predicament is frustrating.
And, once again, as this new lich example demonstrates, you run your monsters as though they aren't just on suicide missions but as though the suicide is their primary objective on the mission. So even after making all of those decisions, you take your last line of defense (tactics) and implement suboptimal ones.
The math of the encounter guidelines and CR calculation are utterly inapplicable to your group's play style. Math will not save you. You are barely playing the same game that page 82 of the DMG thinks you are playing.
This campaign is not a good example of what I'm talking about. But all the problems I've mentioned have happened in every single campaign we've played in, even without potent magic items. The game reached a point from around 8 to 9 where the players' advantages became so great that monsters became fairly trivial to defeat. It didn't matter if it was dragons, undead, demons, or elemental prophets. The game math started to break down in favor of the PCs more often than not at a level I find unenjoyable as a DM because the game seems to be too easy for my tastes.
I doubt very much that your group's style is significantly different in the other campaigns you've played. It has been like pulling teeth over the course of this thread for us to discover the various and sundry ways in which you have altered the game from the one that the encounter math is designed around.
Do you truly believe the encounter math is designed around PC's getting the equivalent of a +1 to every ability score via 4d6 drop the lowest times 7? Do you truly believe the encounter math is designed around level 10 characters possessing
multiple magic items
each that are beyond epic legendary, many of which possess
more power than the PCs themselves do? Do you truly believe that the encounter math is designed around 6 PCs being able to concentrate on buff spells
and other concentration spells at the same time?
And yet you still refuse to appreciate the fact that your PCs are invincible because
you allowed them to be invincible. You blame the game. Your problem has nothing to do with what a marilith's challenge rating should be or what a deadly encounter consists of. The game math is absolutely
not the armor that your PCs are wearing.
I don't get why some people are dismissive of empirical problems with the game math that are not addressed in the monster design in the base game.
There are problems with the game math. High level encounters can be a trick to balance. I just played through a high level adventure the other day where we defeated the 3x deadly encounter much more smoothly than we did the medium encounter that followed it. There was no nova involved because we know our DM doesn't run single-encounter Adventuring Days.
Your campaign is not evidence of problems with the game math. That is why I, for one, am not discussing the game math with you.
They must be addressed by individual DMs, usually by developing some counter to them with custom environments or monsters that put the monsters on equal footing with the PCs when it comes to the base game rules aka game math.
I address it by not instituting a million and one house rules that almost all serve PC power. I address it by not allowing untested material in my games (unless the game is explicitly a "let's just goof off and murder some stuff" type of game, in which case the challenges of the encounter math don't bother me because challenging the PCs isn't the point in that game). I address it by not handing out world-changing magical items like they're candy at levels way earlier than a PC should even
see them, let alone possess them. I address it by running my intelligent monsters like the intelligent monsters that they are. And, so far, the game math has served me fairly well.
Your entire complaint has been "the game doesn't work". But you refuse to recognize the fact that you're not playing the same game as (most of) the rest of us. If all you want to talk about is the encounter math and not the overpowered rules changes and content decisions that you've made, well then of course we can't help you.