Neutral Evil in D&D has always been a bit odd. Is it they don't care about law or chaos and shun both as irrelevant? Are chaos and law branded evil enemies, rivals or stooges? Or are rival evils forces to manipulate in evil schemes? Is NE an older more basic evil?
Chaos can be overly impulsive and irregular while law can be overly rigid. Do NE try and be balanced with law and chaos or just nor care. Maybe screwing up plans of hell and the abyss has made the universe a worse mess and kept the struggle going.
In dnd classic cosmology Hades type underworld is neutral evil but I'm not sure if a gloomy underworld is so evil or a place of punishment like Christian hell is. In my cosmos its where most people end up with extreme alignment fanatics and certain people get dragged to hell or some demon's domain like cattle. Maybe some gods help people to get to certain places but most go to the underworld as gloomy shades and live lives of squalor without descendants giving them offerings or dedicating their mealtime prayers to the ancestors. Spirits really want the smell and essence of the food not the material part. Its ok to eat these offerings or give to the poor or the gods chosen beasts.
The classic Daemons in D&D are kinda ugly jerks, with less famous daemon nobility if any? Daemons, Devils and Demons were all once daimon minor divinities and spirits who took sides in various cosmic wars that shattered the remnant shells of previous worlds which various alignment factions occupied and changed. The abyssal plain is a great maelstrom with small worlds belonging to a demon noble. These worlds orbit outside the world in a great cloud. Various worlds clustered to make the great hells which are adjacent to the underworld and overlap. Much of the underworld is neutral, indifferent or part of a living universe that brings fructifying powers and wisdom to the surface. Many death gods just help the dead or protect tombs they don't especially want more power or to kill people. The underworld overlaps with some other worlds which are not great like the darkness of the outer void that does hunger for life and destruction. There are also planes of shadows and nightmares.
The current structure of the universe makes it hard for beings from the void to scale down into the mortal realm. Some once in our continuum become subject to its laws, adopt alignments and start fighting each other. Maybe it's deliberate.
So my NE plane will be the Netherworld a torturous ruined parody of the world with howling winds and vast dark storm clouds. It overlaps most with the underworld and blends into it growing worse as approached. Most souls brought here don't just come to be recycled as larval evil spirits they come to be annihilated and cast into the void to feed the hunger of the void. The netherworld also overlaps with the negaverse, the shadow worlds and the nightmare kingdoms and acts as a conduit for desperate travellers.
Daemons stockpile larvae, deformed maggot souls of the damned which can be grown into demons, devils and daemons and are an important currency for evil. Daemons keep them mostly to torment devils and demons. Nighthags act as the traders riding nightmares. Daemons farm larvae in great pits and torment them until required. Daemons could make an army rapidly if they needed one and threaten other forces but instead, they lay low and let the old enemies focus on each other and trade with both for profit.
Hell is obviously a prison for the damned and performing a role in the universe while demons tend to be destructive and greedy and their realms reflect their masters will. Daemons in the netherworld besides caring for larvae operate vast prisons for beings from beyond or bad titans who rebelled against the gods. Daemons keep everyone away as many daemons draw power from these thrall powers. Various Daemons serve evil death gods, old evil gods of the outer void and other banished or imprisoned evil gods.
Netherworld Oaths & Gifts
Roll d12 on common oaths and if you get the same one you already had you can pick an oath from this list or roll on Rare oaths. If you have the first 12 oaths you may use the rare oaths table from then on.
d12 Common Oaths
1 Pit law versus chaos at every opportunity
4 Delight in making others weep with despair
11 Be feared not loved
d12 Rare Oaths
3 Make trophies from enemy dead
4 Humiliate the powerless in public
5 Rule over those who fall in your power
7 Plunder resources for you and your kind
11 Lay waste to the land and untouched nature
Bonus Petty Boons for Petty Cultists
Really some people are satisfied with less than an adventurer and don't want to power game through life
d12 Bonus Petty Boons for Petty Cultists
1 Silver Iron blade that harms basic devils and demons and once per day on command becomes 2d4 poison (save halves, has rapid onset)
2 Zombie or skeleton of your very own to command
3 Crime syndicate offers the chance to be involved in a heist for gold
4 Potions 2d4 that blights a common garden area soil for a d4 years
5 Pet snake that steals milk from your neighbours cows and eats hen eggs
6 Evil spoon can poison a drink or meal once per week 2d4 damage and d4 hours nausea -1 all rolls save halves damage and duration. If victim moves more than half speed they must save or stop and vomit
7 Call a poltergeist to haunt a place as long as people have died in the room in the past, a priest may remove such a spirit
8 Acid potion once per month 2d4 save or burns for 1HP for d4 rounds, damage locks or bar or chain
*Petty Missions from Daemons
Daemons are interested in entropy, death, decay and total cessation of everything but the void.
d12 Petty Missions from Demons
1 Feed wild beasts and monsters meat when you can
2 Burn a specific book or collection of documents
3 Frame a demon or devil cult for a horrible crime
4 Look after larvae for a daemon for d4 days
5 Kidnap a person and deliver them to a nighthag
6 Defile a holy place and break its sanctity