Orcus is lord of the undead. He’s a curious sort of demon who resents chaos, preferring a universe in which only dead things thrive. Unlike deities of death who see it as part of the natural order, Orcus bucks the order itself. Hurting living things only somewhat pleases him, a shallow joy that pales against ending life entirely.
I couldn’t find any demon lord connected to more creatures than Orcus, and all of them are undead. In some iterations he tolerates necromancy, but in 5E it’s just a means to an end. He wants his followers to reject existence itself and the merciless gods who imposed life upon it. Unlike most demon lords he doesn’t appear only to the insane or thoroughly evil. Anybody who wants to end their suffering can find their way to him, and he doesn’t hide his solution.
Typical Followers: Undead, full stop. Orcus doesn’t tolerate any followers who expect to stay alive. Ghouls, shadows, and wights are his; these are all undead who actively seek out victims and drain them of life, usually to sustain themselves. A simple zombie is nice, but an undead who propagates is much better. He’s also responsible for certain liches, and though he doesn’t necessarily prefer necromancers there’s plenty of overlap.
Atypical Followers: Most of Orcus’ followers start out alive, they just don’t stay that way for long. His initiates might include anyone from a deposed monarch to a wretched criminal to a hero questioning whether his actions make a difference. Anybody in his cult who still breathes is either in a transitional state or has proven their usefulness, but they’re all a half-step away from becoming something else.
He undoubtedly has followers outside civilization, too. There’s an assumption that most undead are humanoid, but this doesn’t have to be true. A centaur ghoul or an ettin shadow makes for a unique follower, especially if it amasses a following as one of Orcus’ chosen few. He also wouldn’t be picky about creating skeletons or zombies from fallen creatures, and there are as many dead orcs and hobgoblins for potential undead fodder as there are dinosaurs.
Plots: Undead plots, if you’ll forgive the phrasing, have been done to death. Most of them can connect to Orcus in some way, whether it’s the vampire pining for the lost love it had when it was alive or the wraith seeking the descendants of the people who killed it. For a plot more focused on the demon lord himself, treat undeath as a contagion that can be inflicted on others. A single zombie turns its victims into a shambling horde, or a priest succumbs to Orcus’ call and leads his congregation into an evil ritual, or a wizard slips crawling claws into some enclosed space so they can create more victims for her spells. A rare alternative is the destruction of non-sapient life, like the druid who forsakes his old religion and embarks on a spree of deforestation. Orcus isn’t about creating a single undead creature as much as he’s about spreading its gift.
From the Vault: 3.5E has an entire book, Libris Mortis about undead. It is also not especially great; its lore is good, and I’ll defend the necropolitan until my dying day (ha!), but its monsters tend to be either underwhelming or TPKs waiting to happen. I’m more a fan of 4E’s Open Grave, which takes simple concepts like “zombie” or “death-related golem” and expands them into several distinct monsters. For example, consider the skeleton. In 3E, “skeleton” was a template you applied to a creature. In 5E, skeletons are skeletal versions of some existing creatures. 4E held itself to no such standard and instead just came up with skeletons that did neat things. Use one that collapses into a swarm of bones, temporarily gaining the swarm trait, or one who merges with its own catacombs, using the earth glide trait from an earth elemental. An undead creature who seems ordinary but reveals some new, dangerous power is right up Orcus’ alley.
It you want to go totally off-the-wall, I have to bring up the brain in a jar. It’s a sort of lich, a psionically-empowered brain suspended in liquid that usually floats via telekinesis. You could just change a lich’s size and description, but if you want to use a weaker brain in a jar at a lower challenge rating, start with an intellect devourer. Remove every attack except devour intellect and give it fly 30 ft. (hover), immunity to necrotic and poison damage, and the condition immunities of a golem. A more powerful option is a flameskull that deals psychic damage instead of fire, resists non-magical damage, and uses this alternate spell list:
Cantrip (at will): mage hand
1st level (3 slots): charm person, shield
2nd level (2 slots): hold person, suggestion
3rd level (1 slot): hypnotic pattern
It’s missing the standard flameskull’s burst damage, but a brain in a jar isn’t about overpowering opponents. It’s about using them for its convoluted evil plans.