The Abyss is a treacherous place in the world of Dungeons & Dragons. Infinite layers with infinite demons mean an infinite amount of content for Dungeon Masters to build their campaigns and worlds. Many chaotic creatures come from the Abyss, and these are perfect for crafting a story arc around that ends with an amazingly chaotic, villainous boss fight.
Mordenkainen's Tome of Foes provides a deep dive into the Blood War and how it can affect the Material Plane. Each side of the war -- the devils of the Nine Hells and the demons of the Abyss -- sometimes use the Material Plane and its inhabitants as ways to gain ground in the never-ending war. According to MToF, Abyssal demons enter the Material Plane like an infection looking to transform the environment to resemble the chaotic nature of the Abyss. If demons are left to do this, a portal to the Abyss opens, eventually letting the Abyss spill in until that material world is made into an Abyssal colony.
Use Powerful Abyssal Creatures
The final boss encounter can happen at any stage of the Abyssal Infection. Depending on the level of the party, the big bad can be anything from a lesser demon coming to scout the Material Plane to a demon lord leading a horde of demons. For the most dramatic, climactic encounter, the self-proclaimed Prince of Demons Demogorgon is the perfect end-game boss. In this battle, the Demogorgon wouldn't be in its lair in the Abyss, so the challenge of this encounter would be slightly lessened, though the DM is free to use them anyway. The Demogorgon is focused on nothing but bringing chaos and destruction to anything good and orderly. This is the ultimate evil.
No boss fight is complete without minions, and the Abyss sends them out into the world through massive hordes. For this, the "Handling Mobs" section of the Dungeon Master's Guide or Minion rules from Fourth Edition where each minion only has one hit point are perfect for lesser demons such as the Abyssal Wretch or the all-mighty Abyssal Chicken. There are also higher-level demons that pose a good challenge, such as the Maurezhi and the Armanite. A few of those can divert a good amount of damage away from the Demogorgon and keep the battle going a bit longer.
Choose the Perfect Setting to Draw in Characters
The setting of an Abyssal Invasion can be pretty much anywhere. Again, depending on the level of the characters, the seriousness will vary. A small group of demons probably won't do much damage to a city, but a village could easily be wiped out. This location would also need to draw in the characters. A battle with the Demogorgon near an important city would certainly accomplish this goal. This all, of course, depends on the story. If the characters were somehow able to draw the demons away from a city to open their portal, then there would be no immediate casualties. The goal either way is to impress upon the players that this could mean the end of the world.
Such a battle could start below ground in a hidden-away dungeon. This can work as the Demogorgon's new Lair, allowing it to use Lair Actions on Initiative 20. A battle at the end of a dungeon also gives the challenge of a dungeon crawl. If the characters know they're heading in to fight a Demon Lord coming from the Abyss, they will need to conserve their magical and physical resources just to make it to the battle with the Demogorgon. Limiting short and long rests will add to the stakes and increase the sense of urgency. The longer they take, the stronger and more widespread the Abyssal invasion will be.
Create a Perfect Battle Map
A great part of D&D Fifth Edition is the importance of battle maps and how they work synergistically with the creatures on them. The battle map can also be another tool to tell the story. For this, shoot for a large underground cavern. The Demogorgon is a Huge creature, so to give it room to move around among the other mobs of creatures. It also has a swim speed, so another option is to have a large pool of water to give the party a more difficult challenge.
If gaming terrain is accessible, building underground topography with many rock formations to help hide from the Demogorgon's Gaze ability. Creating height and varied landscape in encounters also gives players more creative maneuvering options to possibly gain a strategic advantage against the Abyssal horde. Also decide how the portal to the Abyss is active in the encounter. One possibility (especially if minion rules are in play) is letting the hordes consistently come through until the party is somehow able to shut the portal or eventually defeat the Demogorgon. Should the party fail to stop the Abyssal hordes here, take the fight to the city, creating further tension and urgency to stop the fiendish infection.
