Gritty Rest rules forces you to interrupt the adventure when you run out of ressources. While with the normal rest rules, you just have to wait one day to get your ressources back when you are near 0, with the Gritty Rest, you have to leave the dangerous environment, go back to a safe place, huddle up for 7 days and only than can you continue adventuring.
Gritty Rests mean that the players lose the "chapter" if they run out of resources. Some adventures are single chapter (single adventuring day) and a week break ends them. Others are longer, with a week break baked in.
When you build such a one-chapter adventure, your "budget" is an adventuring day for the party. That is how much XP of monsters you have. It is right in the DMG in a neat table! If you go over it, it means the adventure is harder (if parts can be skipped, you'll want to go over budget to reflect that). If your monsters are all clumped together, that also makes it harder.
PCs who choose to "fight everything" they possibly can could easily run out of fuel, or those who burn resources pointlessly (go nova when it isn't needed).
It is a failure state
on the adventure that isn't "PCs are dead". And the players could decide "risking death is worth it, we move forward despite having no resources left!"
Note that short-rest classes are really key here. In many cases, a night's rest
won't lose the chapter (you should generally make adventures that don't end from a single night's rest). So going back, expending HD and recovering short rest resources is an important "we gotta recover, that sucked" thing.
However, you'll eventually run out of HD and the like.
If you want a harder adventure, consider waiting for the PCs to be higher level.
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Party of 5 level 5 PCs? 5 * 3500 = 17500 XP (adjusted for encounter size) budget for the chapter. For a single-chapter adventure that is all the budget you got.
We then start spawning encounters.
Pairs of monsters get a x1.5 multiplier.
Groups of 3-6 get a x2 multiplier.
Groups of 7-10 get x2.5
Groups of 11-14 get x3
Groups of 15+ get x4
I'll put 1/3 of the XP into a final boss - about 5000ish XP. Make it 2 creatures, so 2500/1.5=1666, which is close to CR 5.
So two CR 5 monsters is the "big bad" of this adventure (5400).
Easy encounters are 1250 xp
medium are 2500 xp
hard are 3750 xp
deadly is 5500 xp
so we end with a deadly.
I want some dire wolves. They are 200 XP each; a group of 6 of them is 6*200*2 = 2400 xp, a medium encounter.
I'll have 3 beats to this encounter -- 3 scenes. The last has the big bad, the other two should have a beat to finish up (a mini boss). I'll use hard budget 3750 each - one has a single CR 8 creature (3900) the other 3 creatures (3750/2/3 is 625 per creature, so CR 3?) 3 CR 3 monsters (700 * 3 * 2 = 4200).
Final fight 2 CR 5: 5400
16 CR 1/8 Guards: 1600
Second scene climax 3 CR 3: 4200
Dire Wolves 6 CR 1: 2400
First scene climax CR 8: 3900
Here is our little adventure. 5 encounters with 3 "big" ones. Suppose it is about a necromancer doing something stupid. The CR 8 is a creature that got loose. PCs are hired to track down where it came from. Dire Wolves are a "random" encounter on the road; or maybe very impressive "guard dogs". The 3 CR 3 are maybe allies of the necromancer (medium scale undead?) which gives the players info about where the final bad guy is.
The final bad guy(s) are at another location and are guarded by 16 humanoid guards (mercenaries) - they are rich. These guards can be killed or avoided or bribed to go away or intimidated or otherwise dealt with.
The bad guy is CR 5, and has a bruiser CR 5 ally. They are up to something bad. Naughy necromancer.
If the PCs give them a week off, the bad guy finishes their necromatic ritual. Most of the settlement dies and becomes a necropolis, with everyone else fleeing.
Now, as we need to inform the PCs of bad things happening if they make a choice - a choice without information isn't a meaningful one - so what I'd do is if the PCs take a long rest, more undead start being spewed by the necromatic rituals terrozing the settlement. If the PCs ignore this and continue their rest, they have been informed that the situation is getting rapidly worse, and chose to do nothing.
If this was a multi-chapter adventure, defeating this necromancer would lead to more information. Tracking down that information would permit a long rest (maybe on a boat, or maybe just downtime as you search libraries).
The disaster from not following through on
that branch could be a month out instead of a mere week.
If I keep leaning in on the "horrible rituals", I could even use the calendar -- there is correspondance with another practitioner, who is going to do the ritual on the Moon of Great Doom and Doominess and Bad, which is 37 days away. There is limited information on who this other practitioner is. This necromancer was trying to do their ritual early, so as not to be scooped.
That 37 day timer starts ticking, and PCs can investigate the other practitioner, run into dead ends, travel, take more than one long rest, etc.
I might write up 3 chapters, each taking ~10 days (well, ~2-4 days plus time for a long rest), to make a long adventure here. Each one would be tied together by a theme and use the adventuring day budget. Introducing travel as needed - boats are great for this. A low tech boat crossing half of the mediteranian takes about a week if it travels 24 hours a day; if restricted to travel during daytime and ties up at night, half that distance.
If I want a really extended adventure chapter I need to use easy encounters for the player's level. Each easy encounter is 1250 XP; an adventuring day with 14 of those is within budget. I could even have 10 easy encounters and one deadly one.
And let me tell you, after 10 encounters "easy" becomes relative.
Most monsters don't have the resources to really effectively realistically do much in a week.
Adventure problems where nothing happens if the heros have a nap for a week aren't much of an adventure.
Know what happens when the orc calls more orcs. Harder fighter ie more Nova.
That's how 5MWD breeds. Punishing it encourages it
If the orc calls enough orcs, it isn't a nova situation. It is a strategic failure.
PCs who choose to charge the horder of 1000 orcs simply die. DM doesn't have to run the combat, just says "ok, new character time".
There are problems "I charge in and attack" doesn't solve.
Only if the axioim is "Players win" is true does letting the bad guys do whatever for a week lead to no consequences that matter. Because there are no consequences.
The point is that with attrition based gaming, you can have players
losing without PCs all being dead. PCs can still die if they don't accept they lost.
And the DM can telegraph this easily without having to say "you have to do X or lose". They can describe the world, and the players can work out "ok, fighting 10 orcs nearly killed us, and there is 100 orcs over there. Maybe we need to run."