To this day I have yet to find a good way of communicating encounter difficulty to PCs, because of how much PC power varies. Like, at level 1, you need to convey to them that a CR3 owlbear would 1-round KO most of them. Then at level 6, a few months later, that same owlbear is basically no threat at all. ...and to most NPCs, all of those monsters are equally "out of my power level".

From a physical description, it's really pretty impossible to convey the difference in power level between a Nothic (CR2), a Spar-Spawn Mangler (CR5), or a Death Slaad (CR10). Stuff like that.

So it feels like I simply need to give the players meta-knowledge and give them info that, out-of-character, makes it clear that "This fight would lead to a 1-round TPK, don't bother" because I don't know how you could convey that with IC info. AT which point it feels like I'm making the decisions for the players.

You could let them make an insight check, possibly opposed by a bluff check from the creature. Success gives them varying amounts of knowledge regarding how dangerous the critter is relative to the party.

In terms of telegraphing... there really isn't a great way. What I have done is try to use some lore. For example, in my Roman Empire game, 5 years before the start of the campaign, the fortress town of Lugdunum was destroyed, and no one really knows how. It was just gone, one day. The Romans sent an entire Legion up the road to get it back, and the Legion never returned. They sent another 2 Legions, and those didn't come back either. At this point the Empire shut down the road and said "no one go up there."

This hinted very strongly to my players, who are very curious about what is going on up there, that this is NOT something for level 1s to do. I just hope they don't think they need to be level 20, or it's going to be a very slow campaign.... ;)

I actually do enjoy reading the discussion over the subject of player death, since I agree with most points from both sides of the fence. Ultimately, I think I've settled on the only right way to handle player death (and the lethality of your game) is by sticking to your guns on what you all agreed with on the session zero. I do prefer not to kill my players since I have a character driven game, but I am curious about taking the kid gloves off for my next campaign.

Quote from Vince_Snetterton >>

Quote from 20kernsd >>

Quote from Wtfdndad >>

Permanent character death is purely DM fiat. End of story. While there may be a limit to what players can do, that doesn't apply to the DM.

I get that you don't like killing your PC's but saying that character death is always DM fiat is not only completely wrong but also with the way you phrase it, it sounds like you're telling other people how to run their game. What if the characters know the BBEG of the campaign is an ancient red dragon and immediately run to its cave at level 1? Now a TPK is always the DMs fault and outside of extreme circumstances shouldn't happen, but not killing a few of the characters in this situation is DM fiat. 

Sincerely, a DM who's only had one PC death and it wasn't permanent.

 I agree with all that you said except one thing. Sometime a TPK is because of incredibly bad gameplay by the players.

True story: Many moons ago I was running a game in  2nd edition. Group was 5-7 9th level players. They were traipsing through the Underdark. They came over a rise and saw in the distance a remarkable thing. There were 20 Githyanki, with 3 Red Dragons as air cover, fighting 20 Mindflayers. I explicitly told the players that none of the combatants noticed them, and the group could simply go around. They thought about it, and decided to attack BOTH SIDES, simultaneously. I then spent 20 minutes sorting the statblocks out, because I thought one one would be that stupid, and this was just a tableau, for interest sake, and I did not need to have statblocks at the ready.

That ended in a TPK, and most of the group was furious I had the temerity to kill them all off. I have no qualms killing off the chars of stupid players. When a DM says "Are you sure?", that DM is already giving a player a 2nd chance. If the player does not take the hint, too bad then.

 Some of us actually have friends that we like to play with so, "killing off the chars of stupid players" is kinda of not a thing at our table, LOL. The question I pose to you: What was the point in fleshing out such a detailed show of force of NPCs only to suggest "simply go around"? You do realize that almost all players hate when a DM grandstands NPCs, allies or enemies. I'm not slamming you but, in all honesty, I have been in a games session or two where we all thought: This is the fight. Only to find out the DM was grandstanding an NPC that we could hardly touch. Your case is much more extreme though, I'll give you that. I don't have lemming style suicidal characters being played in my games so, most deaths are die roll mishaps and a few are honest combat overreach/mismanagement, AKA the party biting off more than they can chew. 

Quote from syzygyeolith >>

I actually do enjoy reading the discussion over the subject of player death, since I agree with most points from both sides of the fence. Ultimately, I think I've settled on the only right way to handle player death (and the lethality of your game) is by sticking to your guns on what you all agreed with on the session zero. I do prefer not to kill my players since I have a character driven game, but I am curious about taking the kid gloves off for my next campaign.

 I think it would be fun to play different game styles as well. The campaign I play in right now is great story and action. The campaign I'm running is going to be dark, intrigue and a combat grinder for survival. Being killed could almost be considered a mercy compared to what some characters might endure.

 Some of us actually have friends that we like to play with so, "killing off the chars of stupid players" is kinda of not a thing at our table, LOL. 

Just because the players at the table are all friends, doesn't mean that players do not sometimes make foolish mistakes or take highly inadvisable actions. If you believe that actions should have consequences in an RPG, then when the players  have their characters do something decidedly foolish, and especially when as DM you have tried really hard to advise them against it, and they do it anyway... well... then there are consequences. I don't think that, as friends, they should get angry about that. Unless you have agreed at the start that there are not going to be any serious consequences to any actions, foolish or otherwise, the players may have their characters take.

I mean even in Critical Role, which I hate to use an example because it is very low, almost zero-consequence D&D, in which the players are allowed to have their characters do almost any crazy thing they can think up without serious consequences befalling them, there was a character death when they did something really, truly, intensely stupid (attacking a high level group of enemies with half their force, and then being dumb enough not to focus fire those enemies one at a time).  And the people at that table are good friends, have been friends for years, and even know each other's children and so forth.

I'm sorry but I'm going to have to reject the idea that if you're friends with someone, as a DM, you can't allow their character to die. I'd argue that if the person is being a mature player of an RPG, that person should accept that there could be consequences like character death... accept that having those consequences be there as possibilities makes the game that much better... and that the DM should not be held "responsible" for negative consequences the player brought on him or herself.

According to the Narzugon monster stat block, who uses a hellfire lance, there is a way to save his character(weather he wants to go back to it or not I'd leave up to him), but he can be brought back with either a wish spell, or if they want, they could track down the Lemure his soul created, kill it, and use true Resurrection on his original body. Both ways could lead to some fun quests. I guess it depends on how much effort the party themselves want to put in to bringing this character back. 

Quote from Wesrin85 >>

According to the Narzugon monster stat block, who uses a hellfire lance, there is a way to save his character(weather he wants to go back to it or not I'd leave up to him), but he can be brought back with either a wish spell, or if they want, they could track down the Lemure his soul created, kill it, and use true Resurrection on his original body. Both ways could lead to some fun quests. I guess it depends on how much effort the party themselves want to put in to bringing this character back. 

 Both of those are 9th level spells, which are well outside the players' reach at the moment.  They'd need to find someone willing to cast either spell on their behalf, and that won't be a free service.

We know from the OP that the player wanted the character to die and replace it with a new PC... but that the player apparently didn't like that the death led to being this devil-creature. So the player might be perfectly happy to know that if the party gets access to Wish, they can get his former character out of the situation.

 Some of us actually have friends that we like to play with so, "killing off the chars of stupid players" is kinda of not a thing at our table, LOL.

 
That’s one of the most insulting and unwarranted things I’ve ever seen on these forums. I’ve been friends with everyone in one of my groups that I run since high school, and I’m literally roommates with most of the other group. Yet their characters die all the time, whether by stupid actions, bad dice rolls, or heroic sacrifices. Because we’re all friends and mature adults, we understand that it’s just part of the game, it’s not personal, and the intensity can make it more fun. I trust a lot of my players more than just about anyone else I know, and I love them, so don’t insult them and our friendship because they enjoy a more exciting style of game than you do.

Quote from Wtfdndad >>

 Some of us actually have friends that we like to play with so, "killing off the chars of stupid players" is kinda of not a thing at our table, LOL. The question I pose to you: What was the point in fleshing out such a detailed show of force of NPCs only to suggest "simply go around"? You do realize that almost all players hate when a DM grandstands NPCs, allies or enemies.

 

Curse. of. Strahd.

I'm actually kind of curious - why did the players jump into that Gith/Mindflayer fray?

One possibility is just miscommunication. DM thought they were conveying "hey, look at these powerful dudes you shouldn't be fighting yet, you can see they've got all these powers you can't deal with yet." Player thought he was being told "look at these powerful dudes you normally couldn't take, but both sides have weakened each other now to make this encounter level-appropriate."

Another is that the player just wanted to get rid of his character.

Or just misjudged the fight level and thought that this was level-approprate for them? ...what were they thinking?

Plus didn't you say they attacked both sides? I mean that is just silly... at least pick a side so you have some allies.

The smart thing to do would be to watch the battle, wait until one side wins, and then, while it's bloody and reeling from the fight, attack that side before it recovers. If they had done that, I could see it. But to attack both sides while they are still currently fighting is pretty dumb.