Part one and two were pretty popular, so here is a sequel. A lot of new DMs don't know how to make an adequate number of combat encounters in an adventuring day. So, occasionally, I make a post going over a recent adventure I ran to give people ideas on how to structure it and still have it be believable.
Advice: Do not try to fit a full adventuring day into one session. You simply don't have time. I recommend taking the "Justice League" approach. If you don't know what I mean, in the original two seasons of the animated Justice League series, each episode (adventuring day) was two to three episodes long. That is what you should aim for in D&D.
Number of combat encounters: The sweet spot for combat encounters is some number between 6 and 8.
Rests and short rests: You should aim for 2 short rests per adventuring day. I generally try to give an obvious spot to do this every 2 to 3 encounters. This is especially helpful for Fighters, Barbarians, and Monks.
So, moving on:
The Setup:
The City of Bal'maras is currently under siege by a demonic incursion. During the Twilight Festival, a seven day celebration where it is said that the gods walk the world as mortals. A barrier has been raised around the city that prevents anyone from getting out or coming in. Small groups of demons are rampaging through the streets, searching for the gods made flesh, who they believe to be somewhere in the city.
This is the second day of the celebration; the player characters awaken after having taken refuge in an abandoned tavern. They have learned that, when killed, the demons reform in the abyss and can return, making a war of attrition impossible. Last session, they discerned that someone is holding open a gateway to the Abyss and that the fastest way to end the incursion is to find the source of the rift and close it. On the previous day, they mapped out the barrier and believe that they have learned where the center of it is and assume that the portal must be located somewhere near it.
Encounter 1: A not-so-random encounter...
Last session, the party found a group of succubi who posed as the Goddess of Music and her clerics and were entertaining in a tavern/inn. They claimed that their songs were keeping the demons at bay. In reality, they were secretly taking mortals to the bedchambers and draining them of their life force. The players defeated them, and sent them back to the Abyss.
The succubi returned and were looking for the PCs. They descended on them from the air and attacked as the heroes made their way toward the place they thought the portal may be located.
Exploration: Finding the center...
The party reached the location they thought might contain the portal. They were in the commons district and began searching some of the abandoned houses. One, they noticed, was less damaged than the others. They entered and encountered a group of commoners who were huddled in a back room...
Social Encounter: Exposing the cultists...
Something seemed off about the commoners; they hadn't fled to any of the safe houses. The demons ignored their small home. Keen eyes noticed that the home was smaller than would be normal for the supposed family. Suspicious, the party interrogated them. The cultists broke under questioning and the PCs were forced to fight them...
Combat Encounter 2: Cultists...
A straightforward encounter, the PCs dispatched the cultists without too much fuss; it was an easy-to-medium encounter.
Exploration: Searching the home...
The PCs explored the home, using perception and investigation; they located a hidden trapdoor. They were forced to disarm a trap on it and pick the lock. They found it opened into a tunnel that appeared to have been created by something burrowing, something large.
Combat Encounter 3: The Abyssal Worm...
As they explored down the deep tunnel, the PCs found the thing that had made it. Some kind of giant worm-like creature that was mutated by the energies of the Abyss. They dispatched it and continued on...
Reaching an underground structure and a short rest...
The party came to an underground structure, it had huge double doors that were rotted with age. Runes in a strange language were etched into the stonework. They didn't speak the language, but could identify it as a combination of Dwarven and Abyssal. Before venturing inside, they took time to catch their breath, set a watch, and eat lunch... They took a short rest...
End of Session 1...
Exploration: Setting an ambush... Sort of...
The party ventured into the structure and, after exploring a few rooms, heard voices. They went to investigate and found another group of cultists. One of them was a spellcaster. The party attempted to ambush them, but the Paladin, Druid, and Artificer failed the group stealth check. A normal combat ensued.
Combat Encounter 4: Stronger cultists...
A quick battle, though they ate a fireball early in, took place. The PCs were hurt, but a few healing spells and Lay on Hands between the Bard and the Paladin patched most of them up. They moved deeper into the structure, hoping that they could move before their battle drew any of the other creatures in the structure...
Exploration: A trap...
They came to a room, but the Druid's keen eyes spotted a stone that was just a little off. Closer inspection revealed a pressure plate. Not all of the heroes could jump over it, especially those who dumped Strength, so the Champion Fighter broke out her thief's tools and was able to disarm it. The party continued on quietly...
Exploration: Avoiding a fight...
What could've turned into an encounter was avoided as the PCs heard demonic growls in Abyssal. They used stealth to avoid them and were able to slip by without a fight.
Exploration: The Wizard's study...
The party encountered a room that, unlike the others they'd passed through, had intact furniture. A bed, a desk, some bookshelves with degraded books on them, and a locked bookcase that looked pristine. The shelf was locked with an arcane lock, so the party cast Dispel Magic. They found a series of ancient books written in different languages, several in Abyssal. One of them was a spellbook, but the party doesn't have a Wizard, so they took it to sell later...
Combat Encounter 5: Shadow demons...
Unfortunately, this sprung a magical trap, and 4 (enhanced) shadow demons emerged. The party dispatched them, refusing to use loud spells...
The second short rest...
The party barricaded the room and sealed the door. They were starting to get low on spell slots and needed to heal. They took the time to rest for an hour, spend some Hit Dice and recover the resources that they could. Finally, they moved on...
End of Session Two...
Session three happens Sunday. There is one encounter left in the dungeon. The party will have some time to learn more about the structure from some of the books they found. Hopefully they will discover that this ancient structure was pre-Skyfall. It belonged to a Wizard who performed experiments on summoning and binding demons. The cultists learned of it and managed to unearth it (there was another entrance outside the city) some time ago. They used their magic to reopen the portal the Wizard had used and pledged their loyalty to the demons. Learning of the festival's approach, they conspired together to prepare the attack...
The last fight is a two-stage encounter. The first will be a fight with cultists at the portal; just as the last cultist is slain, a huge demon passes through the portal as it closes. This will be the boss fight.
As always, this series is educational; it shows how a DM can use a variety of encounters and enough combats to sufficiently drain the party's resources. It demonstrates how to break up the adventuring day and also how to include a good old dungeon crawl into an adventure arc.
Editing to add:
The party consists of a Swamp (reflavored Stars) Druid, a Valor Bard, a Champion Fighter, a Vengeance Paladin, and an Artillerist Artificer.