This is an attempt to avoid thread necromancy, but I wanted to present a homebrew I've made inspired by the original thread.

I'd like some feedback before I offer it to my group.

Combat Exhaustion - 10 cumulative rounds of combat = Check for 1 level of exhaustion.

"The idea is that at the end of ten cumulative rounds of combat, every player must perform a constitution save or gain one point of exhaustion."  (edited for clarity.)
The DC is base 5 (+ Modifier based on damage taken) based off characters lowest HP total during the combat(s).

Maximum hp (undamaged)                    5 Con DC
Maximum hp -1 to 3/4 of max hp        10 Con DC
3/4 max hp to 1/2 max hp                    15 Con DC
1/2 max hp to 1/4 max hp                    20 Con DC
1/4 max hp to 1 hp                                 25 Con DC

ALWAYS rounding in the players favor.

If you drop to 0 hp during a combat you will defiantly take the point of exhaustion, from the moment you reach zero hp no save is available.

If Save roll is a Fumble (ie. a '1' they take 2 points of exhaustion.)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Exhaustion Inspired Spell ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2nd level Spell- Illusionary Vigor                                            Duration: 30 minutes          (Edited to close a loophole)    May only target a willing target

This spell does NOTHING to remove Exhaustion, however it does allow a person to temporarily ignore the
negative effects of exhaustion for it's duration. The downside is at spells conclusion you immediately
suffer (1) additional level of exhaustion.

Thread Necromancy is no longer considered a violation, and as such is no longer discouraged at DDB:

https://www.dndbeyond.com/forums/d-d-beyond-general/news-announcements/2-site-rules-guidelines

Thank you Duly noted.

Quote from Kselyss >>

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Exhaustion Inspired Spell ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2nd level Spell- Illusionary Vigor                                            Duration: 30 minutes

This spell does NOTHING to remove Exhaustion, however it does allow a person to temporarily ignore the
negative effects of exhaustion for it's duration. The downside is at spells conclusion you immediately
suffer (1) additional level of exhaustion.

 

 Needs to be better worded or people will multi cast it and kill something.

Remember that exhaustion in 5e isn't just "a little tired", it means "very, very tired". 6 levels and they die. If your campaign is very survival-heavy it might work, but otherwise...

Also, a couple of things to consider:

1. A save against exhaustion at the end of EVERY combat? I think it should be DM discretion thing instead, or at least "of medium difficulty or more". Because, RAW, your level 20 Barbarian kicked a cat and now they're exhausted.
2. If Berserker Barbarian was bad already, you just killed it. Even if cautious players liked to play it before, no one will ever do so with such a system.
3. This system really favours spellcasters and back liners in general. Sure, pulling a string doesn't seem as exhaustive as swinging a sword, but exhaustion being as dangerous as it is, your melee characters will become almost useless. Lastly, pulling a bow's string might not be so difficult, but casting spells might be.

So personally, I'd change a few things there, but the thought isn't all bad.
Also, why does this really feels like a huge buff to ranged Rangers?

Thank you, I had not spotted that loophole, 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Exhaustion Inspired Spell ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2nd level Spell- Illusionary Vigor                                            Duration: 30 minutes                                 May only target a willing target

This spell does NOTHING to remove Exhaustion, however it does allow a person to temporarily ignore the
negative effects of exhaustion for it's duration. The downside is at spells conclusion you immediately
suffer (1) additional level of exhaustion.

Quote from FireCat5>>

Remember that exhaustion in 5e isn't just "a little tired", it means "very, very tired". 6 levels and they die. If your campaign is very survival-heavy it might work, but otherwise...

Also, a couple of things to consider:

So personally, I'd change a few things there, but the thought isn't all bad.
Also, why does this really feels like a huge buff to ranged Rangers?

Is that so? It's true I didn't notice the word "Cumulative" in there before, so I guess that's my fault. On the other hand...

Quote from Kselyss>>

The idea is that at the end of combat, every player must perform a constitution save or gain one point of exhaustion.

You do say "at the end of combat". Combat doesn't only end after 10 rounds. I don't think I failed to RAW here.

Quote from Kselyss>>

Nowhere does this homebrew discriminate between Melee fighters and other types of characters. Mental fatigue and the adrenaline surge suffered by any combatant can take their toll. After a life and death situation is encountered it is not uncommon IRL for a person to suffer the shakes, caused by a sugar crash brought on by the adrenaline spike.

Again, your failure to RAW is the issue, please re-read the actual text before you decide to declare something broken.

 It doesn't strictly say that ranged characters don't suffer the consequences, that's true, but in the end they're less likely to take damage than the one fighting at the front. Since the DC is based on the % of HP left, those who take more damage will have a harder time to succeed often and thus will gain those levels of exhaustion much more frequently. And sure, barbarians have more HP than wizards, but a fighter using a longbow and a fighter using a greatsword, so long as they have the same CON modifier, will have the same amount of HP.

So yes, both characters will have to roll the save, but the level 1 wizard with -1 to CON who didn't receive any damage only needs to roll 6 or more while the level 1 barbarian with +3 to con, even if he only took a single hit dealing 1 damage, needs to roll 7 or higher, meaning it's harder. Now, for reference, 1 damage is the amount you receive from a cat scratching you. 

Even if I, personally, spend a whole minute (10 rounds*6 seconds) chasing a cat that scratched me slightly, I would be, at most, tired for a minute. And I'm no superhuman.

It's a bad idea, it incentives players to rest all the time. That's beside the fact that humans have some of the most outrageous endurance in the animal kingdom, the average level 1 adventurer starts with Olympic levels of potency, and fighting might be exhausting, but not in a world of super humans that can match feats of reality warping magic with unreal feats if power. 

I lived with a super marathon runner, he did 50k double marathons, he broke down on the 100k super marathon. All of your physical warriors are superior to my roommate. 

Quote from FireCat5 >>

Is that so? It's true I didn't notice the word "Cumulative" in there before, so I guess that's my fault. On the other hand...

Quote from Kselyss>>

The idea is that at the end of combat, every player must perform a constitution save or gain one point of exhaustion.

You do say "at the end of combat". Combat doesn't only end after 10 rounds. I don't think I failed to RAW here.

You are correct, my intention was at the end of 10 cumulative rounds of combat, my casual use of end of combat can easily be mistaken as meaning every combat by a casual reader. That should be an easy edit.

Edit: "The idea is that at the end of ten cumulative rounds of combat, every player must perform a constitution save or gain one point of exhaustion."

Fixed, and Thank you.

 

Quote from Kselyss>>

Nowhere does this homebrew discriminate between Melee fighters and other types of characters. Mental fatigue and the adrenaline surge suffered by any combatant can take their toll. After a life and death situation is encountered it is not uncommon IRL for a person to suffer the shakes, caused by a sugar crash brought on by the adrenaline spike.

Again, your failure to RAW is the issue, please re-read the actual text before you decide to declare something broken.

 It doesn't strictly say that ranged characters don't suffer the consequences, that's true, but in the end they're less likely to take damage than the one fighting at the front. Since the DC is based on the % of HP left, those who take more damage will have a harder time to succeed often and thus will gain those levels of exhaustion much more frequently. And sure, barbarians have more HP than wizards, but a fighter using a longbow and a fighter using a greatsword, so long as they have the same CON modifier, will have the same amount of HP.

So yes, both characters will have to roll the save, but the level 1 wizard with -1 to CON who didn't receive any damage only needs to roll 6 or more while the level 1 barbarian with +3 to con, even if he only took a single hit dealing 1 damage, needs to roll 7 or higher, meaning it's harder. Now, for reference, 1 damage is the amount you receive from a cat scratching you. 

Even if I, personally, spend a whole minute (10 rounds*6 seconds) chasing a cat that scratched me slightly, I would be, at most, tired for a minute. And I'm no superhuman.

It is only checked for after a cumulative 10 rounds of combat, in my experience if a single combat lasts 10 rounds it was a very challenging fight and would have drained the casters reserves and they'd be looking to refresh their spell slots with a rest anyhow. The DC only becomes arduous if you were wounded and in line with how sever the wounds. In my opinion it does not incentives any additional rests. However, playtesting may prove me wrong.

If your marathon running friend, had been receiving the lumps and bruises experienced by an Olympic boxer in 3 minutes of boxing and one minute between rounds that might be a valid equivalency. By round 3, most boxers are experiencing or closes to experiencing a bit of fatigue. Professional matches are limited to a maximum of 12 rounds, an average match only goes 6 rounds.

The Idea was to raise the bar making combating with the superhuman characters seem more realistic, more dramatic and add tension as per the posts title:

"Idea for combat exhaustion to make combat have more tension".

It is a homebrew to serve the stated purpose, I believe it does that admirably. If it is not to your tastes, don't use it.

There are crazy Russian medieval MMA matches on Youtube to browse: https://www.youtube.com/watch?v=uGlDFrrZFrE&t=51s that show just how quickly you can burn out wearing combat gear. Note that around the 1 minute mark, these guys are already getting pretty tired. You might say 'well, they're using blunted weapons, that doesn't count', but in practice, it is really hard to get into the the little spaces that armour doesn't protect, because the one wearing the armour knows where those vulnerable places are and will fight to guard those places. You're GOING to have to bash at the protected places in an attempt to soften your target up to make them flinch and get to those vulnerable spots, not to mention defend yourself, so the sword arm of the average fighter is going to get pretty tired pretty quickly.

Now, that said, adrenaline offsets fatigue really effectively. Being in a boxing match is a far cry from fighting for your life. A boxer might be tired after the 3rd round, but if his life depended on it, you can bet he'd be able to move at full speed without issue. Look at stories of people who have survived shark attacks, missing part of their leg and suffering tremendous blood loss but still able to swim to shore and THEN climb rocks to get to a road to get help.

Rather than by turn, I would think it might make more sense to think of combat exhaustion in terms of complete battles. As you said, it's rare to have combat last more than 10 turns and even then, that's only a minute in real-time. Adrenaline lasts longer than that. In that case, the party may have had to go through 4 or 5 skirmishes and you can bet they would be dead on their feet after the 5th one if they had to contend with enemies with a numbers advantage constantly. Given the average 6-7 turn fight, maybe after the 4th battle without a rest, that's when PCs need to make their first exhaustion save. The individual battles would probably last less than 10 turns, but without being able to take off armour and rest properly, exhaustion could easily slip in and get exponentially worse. Cautious parties would be forced to consider retreating and having their nemesis bolster their defenses for the next attempt or soldier on and use sneaky tactics while avoiding combat as much as possible until they can catch their breath without retreating.

I think part of the issue is also in factoring in exactly what's needed to have a 'rest'. "Let's stop here for now and catch our breath" is a far cry from being able to take off combat gear and let your body recover. Think of all the crap adventurers lug around all the time, that alone would be enough to tire most athletes out if they had to carry it all damn day. If you're still carrying weight, you're not resting, and in a perilous area, taking off armour or putting down weapons and being left vulnerable is a big risk, which could be a way to punish slothful players trying to metagame; if the party is crawling along and resting after every skirmish, that's essentially giving everything else in the area more time to get a fix on where the party is and hunt them down in greater numbers.

Thank you KinaJardine, I can see how the interval might seem too short, and am willing to consider upping it more in line with your suggestions once I hammered out the mechanical aspects of the system, (after all that is what playtesting is for) details of how quickly to check for exhaustion, penalties based on what percentage of damage suffered, etc. are all on the table to be zeroed in after observing how a workable base system functions in play.

My purpose was to generate a workable first draft, and throw it out to have help spotting the obvious poor wording and exploitable loopholes, the feedback so far has been great for that.

As for how long counts as a 'rest'; I was going strictly by the one hour of downtime called for in a short rest the basic rules cite.

"A short rest is a period of downtime, at least 1 hour long, during which a character does nothing more strenuous than eating, drinking, reading, and tending to wounds."

This downtime does bear inherent risks as you've stated, and resting in the heart of enemy territory should be reserved for the foolhardy or the desperate.

Okay, after doing some random character generation, here's what I've found:

Characters carry a LOT of crap around. Like, your average army pack is about 60 pounds. That includes your body armour, helmet, provisions, ammo, field tools, etc. Most soldiers can march a full day carrying all this around.

I made 7 different characters and the starting equipment tends to average a whopping 80-100 lbs. That's a LOT to be lugging around. Now, obviously, as soon as combat starts, you're going to shed your pack and just have your weapons on you, but if you want to ratchet up tension and realism, I'd take aim at carrying capacity for determining just how quickly exhaustion sets in. Like, if you've been on the run from enemy patrols or hunting parties, you're not going to be able to carry much aside from armour and weapons and keep up a good pace. Even strong characters will only be able to keep hauling all that stuff around for so long. Now, that could just add a lot of extraneous rests, but suddenly it DOES make your characters way more mortal. They get tired from trying to haul all their equipment around. They might even require the use of a pack mule and have to worry about keeping it safe and healthy. They'll have to pick and choose what treasure to take and what they'll have to leave behind. It will be extra work for the DM, too, to keep a mental note of how long your players have been traveling carrying around an entire department store on their shoulders, but could also lead to some opportunities to have those tense moments where the party is just getting worn down by constant small skirmishes keeping the party from getting the rest they need. Exhaustion is a very under-utilized mechanic that typically only comes up in long, drawn out dungeon crawls, and really should be a much bigger factor in a lot of cases, and your thread got me thinking about that. Whatever you come up with, I may just have to steal use.

That is an awesome addition, making carrying capacity and encumbrance matter. The whole reason I wanted to incorporate such a mechanic was that adventures are often light hearted fantasy romps. And by all means those style of games are wonderful entertainment with great escapism for players who want to keep things rules light.

As a DM, sometimes I have players that want a more gritty and grim-dark world, where every decision matters. If I as DM am willing to assume the burden of tracking combat rounds and carry capacity to provide that then I am providing my players a world where each victory is more worthy of a Conan, or a Khal Drago.

In your opinion do you think adding a bonus +1/penalty -1/-2/-4, to saves as written for those approaching (under 50% / over 50/75/90%) of their carry capacity, and checking for exhaustion every 6 hours just from lugging their kit? Those OVER carry capacity (ENCUMBERED), check every hour with the -4.

I think it would also increase the spell value of spells that are currently underutilized. Such as Floating Disk, Catnap,  & Tiny Hut.

Definitely, that sounds reasonable. If you're hauling around something you have trouble LIFTING, you're going to be utterly spent after an hour and only the most stoic of adventurers would be able to keep going much longer. Encumbered adventurers should be basically inviting disaster, not minorly inconvenienced. Moreover, once they hit level 2 exhaustion, that's it, they cannot keep carrying that weight and move without dragging. The decision to leave some stuff behind MUST be made.

50% carrying capacity, that seems to be right where the military keeps its benchmark at for marching. I would call that 'traveling lightly' because even the average joe can walk under load without too much issue as long as the weight is properly distributed. Like, they'll be tired, but not 'I can't go on' exhausted. I would rate 50% (rounded down, for the ease of math) as not really needing to make extra exhaustion checks unless they've got to deal with difficult terrain. I'd mark 75% as the point where they'd need to make saving throws every 6 hours and within 20 pounds of their maximum where they save at -2, and encumbered characters saving every hour should be sufficient to get some players to lighten their load already. -2 every hour pretty much guarantees the onset of exhaustion after 3-4 rolls, probability will do the job just fine.

I see what you're trying to do here and I appreciate the realism, but accumulating penalties in a battle that's already a grueling slog could be just rubbing salt in the wound of a poorly designed encounter. There's a thing called a Death Spiral where being penalized for already doing poorly causes you to do worse which causes you to incur more penalties until you end in a TPK. Death Spirals spawn feelings of helplessness and resentment rather than positive dramatic tension. I think you could preserve the tension you're going for without ramping up frustration if you had a more robust mechanism of delaying the exhaustion check.

Harkening to the Warlord from 4th edition, it would be cool to let the party leader make a rallying cry that postponed the exhaustion check, or a victory yawp when an enemy is defeated. Thus staving off exhaustion becomes an active pursuit. I'd probably have these be checks with escalating DCs as you've already worked out rather than spells. After 10 rounds of combat, how likely is the caster to even have a slot to cast Illusionary Vigor?

With something like this, you could actually move up the exhaustion point to something like 5 rounds with a few encouraging words extending it fairly easily. This would at least make the concept more visible and more often used so it's not just busted out in fights where the players may already be fairly fatigued and/or grumpy IRL. It could also serve as an early warning to take stock in how the fight is going to maybe determine if you need to retreat or shift into higher gear to finish the fight quickly.

One other warning I'd give about 'gritty' houserules is that people often add multiple ways to get exhaustion without realizing how deadly it becomes when it stacks up. If you're gaining 1/day from the harsh environment and 1/day from combats and 1/day from encumbrance or whatever, the party quickly becomes nonfunctional. You can often end up with days so punishing that the party will want every long rest to last 3 days. And if you say no to that they could very likely just die. I'd start out with one new mechanic and slowly ramp it up from there as you see how the effects play out.

All very well reasoned @Scatterbraind. I had failed to even consider the difficult terrain aspect being cumulative with my mechanic.

I definitely want to avoid the pitfall of this mechanic causing a death spiral. 

The addition of character based solutions that can stave off a check can become invaluable, and if each solution is only a  'May only benefit from this spell/ability 1x between long rests' to offset exhaustion' means a well rounded party can stay viable much longer than a party not constructed for the long haul.

Maybe allow inspiration points, to buy off exhaustion on a 1:1 with a limit of once per character until after they've taken a long rest?

Defeat of a challenging encounter allowing use of a (maneuver) Victory Yalp that can delay the parties need to check for another 5 combat rounds. 'May only benefit from this spell/ability 1x between long rests' 

Bardic "Encouraging Words" could net the party another delay of 5 combat rounds. 'May only benefit from this spell/ability 1x between long rests' 

The Druidic "Natural Vigor" or Clerical "Divine (or Infernal) Stamina" allow you to skip a singular exhaustion check between long rests and are for parties of up to four, using a higher slot allows you to add 2 additional people per higher slot.

With little effort, a (sub)class specific spell or ability could be designed for each class to slow the descent into the exhaustion spiral.