Yeenoghu is savagery incarnate. He divides all things into two categories, predators and prey, and he is the alpha predator. He has no love for the trappings of civilization, and his perfect world is one in which everything succumbs to its bestial nature. He seems like a boring, one-note monster, but oh wait we already did this.
Yeenoghu is what you get if you start with Baphomet and take away everything that makes him interesting. There’s no cunning behind his actions, just mindless destruction. There’s no subtlety in his worldview, just malice. There’s no interesting race associated with him, just near-mindless creatures for PCs to kill without thinking. Everything about him is somehow both effortless and tedious, and I see no way to turn such a basic, tired concept into a worthwhile element in a campaign. If you really like Yeenoghu, re-read the Baphomet post but substitute “gnoll” for “minotaur” and take out anything that suggests he has any sort of patience or cognition. I’m instead using this time to talk about a demon lord from a previous edition who I think deserved to be in 5E more than Yeenoghu did:
March of Madness: Pale Night
Pale Night is older than demonkind itself. Purported to be from the race who filled the Abyss before demons swarmed over it, she waits in silence, a mystery to all. Some say she created several of the demon lords who threaten the universe, and others say her influence is better felt in the creatures she set loose on the Material Plane. Even her true form is ambiguous, covered as it is by mask and fabric, and most see fit to give her a wide berth, knowing her only from the numerous, often contradictory tales told about her in books of lore and fables.
Her immediate goals change due to circumstances known only to her. On one day she may stage an invasion of an elven kingdom, on another she may warmly greet travelers who trespass in her domain, and on a third she may seal herself off and destroy any who so much as say her name. Only two things remain constant: she delights in magical and biological experimentation, and she wants to purge the world of demonkind so her race of old can return to prominence. Until that day, she’s content to use demons as pawns.
Typical Followers: Few choose Pale Night. Her cults are small and rare, even more so than those of other demon lords, and they consist almost exclusively of spellcasters who share her love of experimentation. She offers no boons except a touch of her influence in their work.
Monsters, however, owe a great deal to her. Her favorite trick is combining two creatures into one, and many common monsters on the Material Plane may actually be the results of her actions. You can change any such creature’s lore to include her; obvious candidates include harpies, owlbears, perytons, griffons, and bulettes. Though not all of these creatures are capable of worship, a follower of Pale Night may claim them among his allies or protectors.
Atypical Followers: Because of her seclusion, capriciousness, and narrow focus, only rarely does a person even hear of Pale Night. Truly dedicated scholars may come across scattered mentions of her, and the lure of an impossibly ancient source of information can prove too strong to ignore. In extremely rare cases a paladin or similar agent of good may ally with her followers to fight their mutual enemies, the demons. And her predilection for transformation may find allies in strange places, like the barbarian willing to do anything to grow stronger.
Plots: You can explain almost any custom monster as one of Pale Night’s creations, whether it’s picking off travelers outside a trading post, living in a castle as a king’s trophy, or guarding a cave so the druid inside can work in peace. It’s better if the monster’s origins are unknown, which could trace back to one of her cults or an enterprising fiend working in her name. Because her place in the Abyss is not well-documented, characters may find themselves in her realm by total accident, or they may intentionally visit to search for something lost for millennia. It’s especially meaningful if they seek knowledge; Pale Night has a unique perspective on history and a long memory, and she may be willing to part with any information for a sufficient price.
It’s also said that even glimpsing Pale Night’s true form can drive a person mad. This could present as the madness of another demon lord until the players discover the truth.
From the Vault: Templates, by and large, did not survive the transition to 5E. A template is a set of rules you can apply on top of a creature to make a slightly different creature. One of the few templates we have is the half-dragon, and we can modify this to create a half-fiend template representative of Pale Night’s experiments. Remove blindsight and add resistance to cold, fire, and lightning instead of using the table. Add immunity to poison damage and the poisoned condition. Use the Abyssal language instead of Draconic, and instead of the breath weapon add an attack from a demon of a challenge rating comparable to the base creature. This may require some tweaks to the base monster, but that’s the point; a giant ape with a chasme’s proboscis attack will look horrifying, exactly as one of her creatures should. If you want you can add additional features from demons, like flight or magic resistance, but try to keep them to a minimum. Too many changes can do unexpected things to the monster’s challenge rating. You want to unsettle your players, not kill them accidentally.