I would honestly love to play a game like that. I would have to learn so much both as a DM and as a player as to how those systems work. And even then its not like It would be devoid of situations. You'd have to fend off hunters and predators. You'd have to worry about the politics of owning land and running a business. If you get bored you can just kill all your cattle, sell off their parts and move on with your life having made a pretty penny. 

I’d say do one of three things:

1. You could try the one shot idea that that one guy had

2. Add new players to the group that might pull the group toward adventuring again.

3. Just talk to them and explain your situation. They should understand

Stakes.

If they be running away then nothing is happening that they care about more than they fear. 

Make consequences for running away/not engaging/putting off responsibility and you'll give them the opportunity to be brave.

What if the monster that they ran from was heading straight for an orphanage? What if because they didn't investigate the dark magic then innocent people died?

And if this doesn't motivate them, keep trying until you find what does. And if they keep refusing adventure even then... let them. Let them do nothing. Let a year pass. Just keep asking them what they do. No one approaches them with quests now because they're has-beens. They're going to be have to get jobs.

Ok now I'm really fascinated in how that would play out. 

Generally our job as DMs is to find out what our players enjoy and do that BUT it goes both ways and if a group is boring you, it's totally ok to let them know and cancel the game. Life's too short for average games when this hobby can be so great.

I think in general, raising the stakes and making there be real, very negative, consequences for their cowardice is good practice.

However, I still think this needs a conversation with your group. If their motivation for running away is to "not have something bad happen" to their characters, and you make bad things happen around their characters, because of the failures of their characters, the players are going to be even more miserable than before. They may quit the campaign.

I think you really need to hash this out with them, and be very clear as players and DM what kind of game with what level of consequences there may be. For instance, I have been very clear with my players -- they are welcome to ignore whatever plot hooks I put in front of them but don't expect the world to stay static. Their Empire is under attack. Already while they were dong something to help the empire, another city halfway across the nation fell to an enemy who is still somewhat unknown to them. If they ignore what is going on in the world, this may keep happening until their comfy home Empire falls and they are enslaved to a merciless enemy (or any number of things). But my players think this is cool, so it's OK.

You have to make sure the players are not going to freak out if you have the town overrun by zombies because they left the town for other people to save, and those people weren't strong enough. Yes, it is a valid consequence, and the PCs absolutely deserve it for being cowards. But if the players hate it, you won't get anywhere as a DM. You'll just end up causing the campaign to implode.

Maybe inject a half-joking encounter with a talking lion who passes hem the gift of courage?

As a follow up, I had a conversation with my group.  It breaks down like this...

- The 'leader' has a borderline insane belief in his character's abilities.  He is a level 9 rogue, but he believes - absolutely - that his character alone can one-shot any adventure/campaign/god/etc by just sneaking in and murdering everyone in their sleep.  Nothing I argue can dissuade him of this notion, and he has indicated that if we tried it, and he failed, it could only be because I fudged the dice rolls against him to try to prove some evil dungeon master point.  This is a whole other thing... I don't quite know how to deal with yet but it leads to the root of the present problem...

- Because the 'leader' believes his character is an unstoppable stealth killing machine, he (i think both the player and the character) feels guilty dragging the other PC's into danger.  Like he is Superman, but he keeps having to bring Lois and ... Jimmy?... along with him on missions and putting them in harms way.  So he is over-protective and pulls the plug on combat before anyone can get hurt, believing that his character can just go back under cover of darkness and solve all the worlds problems with his murderknife.  But he doesn;t actually try that, we just move on to the next adventure.  And my other two players are very passive, and agree to abort mission once the 'leader' says so.

I'm sure there was some delicate, psychoanalytical way to address this, but I just told them all I am not writing homebrew adventures with my tiny amount of free time as a working father of two, for them to run away from every story.  So either start adventuring and death-defying, or I will make them trashcan these character and roll up new ones who will fight, or we can all quit.  So far so good they have found their fighting spirit. 

But I am thinking for the long term good of the game I may need to kill off the leader... (the character, not the player).

You could try an in-game holodeck-style adventure simulator, where the party can test their fantasy without the actual risk of consequences. Perhaps a wizard wants someone to test his security system before actually investing in it.

Within the simulator, you can do all of the rolls on the table, and even hand them the monster statblocks, since their host is encouraging them to try to break it.

Let them fight something that is tailor-made for their weaknesses.

I wouldn’t recommend targeting him specifically, then you’d be making his conspiracy fantasies about you being an evil dungeon master come true. 
That said, maybe have him make some will saves and see how he likes being dominated or hit with a fear effect. Or have him run into a fighter that loves to grapple, and see how well he can melt into shadows when he can’t move. Not to kill him, just to make a point. 

It's good that they are fighting now. It shouldn't take too long, with just general gameplay, for him to find the shortcomings of his rogue. mix in the odd enemy which will cause him issues - add a potted Shrieker to alert people of his presence, and so forth. 

I have 2 points: 

The first is you might want to give the players some variation on the "the worst thing that can happen in D&D is not that your character might die, the  worst thing that can happen is that we sit around a playing for 4+ hours and nobody has fun" talk with them. 

That brings me to my next point though: if the characters *are* having fun and just prefer this style of play, then I think they might have a better time playing another game. There's hundreds more RPG's out there and many of them are much less combat focused or rules crunchy than D&D, and it's fine if that's more what they want without knowing it. I'm specifically reminded of Call of Cthulu, where your party's paranoia would be COMPLETELY justified, because it's a game about paranormal investigators who have to be super careful or else risk losing their sanity or their lives with one false step. If you get in a fight in CoC, then something horrible has happened and you should start thinking of your next character.  Or, on the less severe end of the TTRPG spectrum you have games like Monster of the Week that are all about planning and pooling resources and preparing to fight monsters (that will absolutely kill you without taking precautions) within more of a narrative framework rather than a rules framework. Adventure Zone season 2 is played in that system and does a good job highlighting the differing play philosophies between it and D&D in the early episodes. 

TL;DR: either your players are afraid that losing a character means losing the game and are afraid of that, OR maybe they'd just prefer one of the many other RPG's our vibrant table-top landscape has to offer. 

*EDIT*

Just skimmed back through and saw the comment of the rogue who believes they're somehow superior from the other characters. Why do they think this? Do they believe that the developers would create a game so obviously unbalanced that can be so heavily exploited? 

It's funny you bring up Cthulhu.  That's where all of our backgrounds are.  This is my first D&D campaign and I think only one of my players ever played D&D and that way Greyhawk back in the late 80's.  D&D was decided upon because they wanted a game where they could fight the monsters.  

But I had to kick a player who just got too disruptive and another left as a result... and I am left with the current group that decided to run away from fights.  But I think we are good to go now after a little pep talk.

RE: the rogue... I have no idea.  There is no rational explanation for why he thinks his character is an unstoppable, undetectable murder machine.  A couple of contributing problems:

- He has acquired a couple of magic items that aid stealth (AID, not "allow for perfect undetectable stealth 100% of the time").  But the player has head-canon for himself that a character with his skills and these items could never ever be detected ever under any circumstances.  And he complains when this is shown to be false... but he accepts it because "I get it... you're the DM and you have to break the rules so my awesomeness doesn't completely break your game."  Despite the fact that his head-canon is what is inconsistent with the rules.

- He believes any surprise attacks he makes should be an insta-kill.  He is always saying things in combat like, "For my sneak attack, I cut the boss' throat.  He is dead."  I even tried homebrewing some rules so he could try for kill-strikes at higher AC or at disadvantage but he hated it (because he felt he had already earned an insta-kill with his successful sneak attack).  So he "accepts" that I am "arbitrarily" limiting his character.

- I think the player has gone a little crazy during the pandemic and this is a way it is manifesting. 

Despite how the above may sound, it isn't all that disruptive.  He defers to my authority as the DM, he just thinks I am bending the rules to limit him but accepts it because he is a team player.  So I just quietly roll my eyes and thank him for being sooooo understanding.

So when you show the rogue's player how the actual rules for sneak attacks and the magic items ... what do they do?

Regarding their "called shots" I'd explain it like this, "Ok, D&D just doesn't give the player that level of control over the damage they inflict.  I appreciate you wanting to add some detail into your actions, so how about we describe exactly what your character does to their quarry _after_ we roll the damage, then you can color it anyway you want."  With the magic item protest, that's where you say, "I'm sorry, I think you don't fully grasp the rules of the game.  These pages tell you how the item works to assist you but not make you omnipotent.  I'd appreciate it if you'd stop insinuating that I'm bending the rules to "get at" you.  That's not the dynamic I want to play and you creating this myth is likely having a detrimental effect on other players with you forcing the perspective that this is some sort of antagonistic game between us.  I appreciate your desire for your character, but at this level they just aren't there."

Maybe they're layering one of the CoC "lethality" rules over their D&D and don't realize it.

There is also the concept of "Whatever the Players can do, the NPCs can do as well", which is implicitly true.

Regardless of how powerful any player may think they are, the fact that the party hasn't encountered someone with the same apparent lethality is purely a demonstration of the DMs mercy.

Introduce him to the Clone and simulacrum spells. He can be as stealthy as he wants, but if he doesn't get the right target, or the target can resurrect, then he's screwed when the botched attempt seeks revenge.

If you are all from Call of Cthulhu, then the players need to be clearly told that monsters in D&D are NOWHERE NEAR as scary as the ones in CoC.

PCs should be able to deal with the same number of monsters as PCs without coming close to death (of course, unless the DM sends 3 dragons against them!)

I would like to add an idea maybe you could do it since they like commanding people to fight for them to use that as the campaign like there military general or something to that idea so their character is protected but they can still fight with NPC from safety so they can get used to combat or they control a few of the soldiers so they can be ready to fight with there a character from a distance. (The more I think about this might be an interesting campaign to run on its own.)

How can the party think you're cheating if you roll in the open?

Is the rogue an assassin?

Quote from Wysperra >>

How can the party think you're cheating if you roll in the open?

Is the rogue an assassin?

 I got the impression that it was because he didn't understand the rules properly. So saying something like "I slit his throat" should be a instant kill in real life, but as has been said many times - D&D is a game, not a simulation. In D&D you get various extras (automatic critical, advantage on attack roll etc) but not an instant kill. 

Maybe play a game with just him, one session and let him do the sneak in thing. Have the rules prepared on cards or something so you can both review them when it comes up. Then there is no hope that you have to worry about him "overshadowing" anyone. 
Guy sounds like a bit of a condescending jerk, but we are only getting one side. 

Quote from Okahey >>

Quote from Wysperra >>

How can the party think you're cheating if you roll in the open?

Is the rogue an assassin?

 I got the impression that it was because he didn't understand the rules properly. So saying something like "I slit his throat" should be a instant kill in real life, but as has been said many times - D&D is a game, not a simulation. In D&D you get various extras (automatic critical, advantage on attack roll etc) but not an instant kill. 

Maybe play a game with just him, one session and let him do the sneak in thing. Have the rules prepared on cards or something so you can both review them when it comes up. Then there is no hope that you have to worry about him "overshadowing" anyone. 
Guy sounds like a bit of a condescending jerk, but we are only getting one side. 

 Are we sure he seriously believes this? I know a lot of people try at one time or another to play a character that is delusional or narcissistic (or both), are you certain he actually believes that nobody should ever be able to lay a glove on him and he's the most perfect sneaking machine, or is he just having a lot of fun roleplaying a delusional character and pretending to believe his characters delusion in what he thinks you know is a running joke? 

That's the only other thing I can think of for why someone with any degree of access to the actual rules of the actual game would have his head so firmly in the sand about it.

As part of the discussion, you need to stress that "XP is for closers".

If the party runs away and recruits town guards to fight the battle then the players get no XP. After all, they didn't overcome the encounter.

They also, of course, get no treasure. The town guards keep it all.

As a GM, be blunt. Explain that your job is to be a champion of the characters. You are there to provide challenges that are hard, so that characters struggle to overcome them, but not too hard, so that characters can overcome them and the players feel like badasses. The players have a part to play in this - to actually attempt the challenges. If they have their characters sit in a tavern all days, that's boring, for everyone at the table.