Every DM has their own perspective on this question:
do you play the monsters to win?
The thread about how often a PC death "should occur" has had myself thinking on this for a while now. I know, as DM, I can throw whatever I want at the PCs, and while I don't think success should be guaranteed to the players, I do feel the point is generally to make certain they have a fighting chance--perhaps even a fair one on occasion.
But how cunning and resourceful should their opponents be? What lengths should they be willing to go to defeat the characters? (dependent on their personality, alignment, behavior, morals, etc. of course)
How much foreshadowing should the DM feel necessary to warn the players, "Hey, this is going to be a tough fight, perhaps a TPK, so I'm giving you hints of a warning here..."?
While "not fun" for the player in question, one tactic is using magic or terrain to isolate one PC from the party; either to deal with that character or remove them from the battle while the other PCs are dealt with. This typically greatly increases the monster's chances of defeating the party (the "win").
Another strategy is using a familiar or similar creature or magic to spy on the PCs to observe their strategies, strengths, and weaknesses. This allows your monster to gauge which PC is the greatest threat and in what fashion, so they can counter the PCs as well as possible.
When the adversaries are multiple casters, such as a Hag Coven, this creates situtations which some players might feel are "unfair" and certainly "un-fun".
For myself as a player, since I often DM, this has never been an issue for me. And while I feel a primary goal of the game obviously is for
everyone to have fun, I sometimes find myself at odds because what is fun for myself as DM is a believable world which challenges the PCs, not a game where the players just get to enjoy themselves and "win" all the time.
The last thing I enjoy and very rarely ever do is bail the players out of the situation. I try to give them warning, the chance to learn what they can instead of just charging blindly ahead, retreat (to a point---sometimes you are just in too deep!) and regroup, etc. but if they fail to take advantage or think of such things for themselves, I often feel like they have no one to blame
but themselves.
I know the battle looming ahead for my party looks bleak for them from my point of view. While well-equipped and working well as a team, I see the potential for a TPK, particularly if I separate the PCs from one another and deal with them as smaller units instead of a whole group.
Weeks ago, I posted about the dragon encounter the PCs faced. Even though only 7th level at the time, the managed to "fend off" an Adult Black Dragon (CR 14), at the loss of a party member. Through some luck and planning, they managed to avoid the second encounter as the dragon hunted for them after resting. So, perhaps this encounter will go better for the players than I am imagining. We'll see.
Regardless, I am not really looking for "answers" or anything, other that what you might care to share from your own persepectives or opinions.