Orcus (MToF)
Orcus, the Demon Lord of Undeath, wants to kill the multiverse and replace everyone with undead creatures under his control. For a Demon, he is actually rather productive, providing the rituals that create Liches, Ghouls, Bodaks, and numerous other forms of undead. He can even turn Demons into Undead, which is still mind blowing for how metaphysically impossible that should be.
It is safe to say he is the hardest working Demon Lord. Starting off as a meager Larva and rising through the Demonic Ranks. You could further say that of all the Demons, he would be the one who could transcend being a Demon, and you would be totally correct, he did. Shortly after attaining godhood, the other gods killed him. Naturally, being so tied to undeath, he got better. But the loss still cost him his divinity, which is rumored to still exist somewhere out there in the multiverse, waiting to be claimed.
You could argue that Cultists of Orcus are a nihilistic sort, but that’s not entirely true. They have a goal in the form of their ideal world of undeath after all. What is true, is that they want to kill everyone to achieve it. Often in less than pleasant ways. Cultists can gain a power that is slightly worse than Relentless Endurance, allowing them to play Whack-a-mole like a PC. Cult Leaders have an AoE Aura that inflicts disadvantage on death saving throws, further encouraging that style of play.
Orcus has his own Abyssal Lair, the fortress Naratyr in Thanatos. While in his Lair, he can use Power Word Kill on any creature inside the lair that he is aware of, he doesn’t even have to see them. Alternatively, he can raise a squad of undead, or cause the body parts strewn about the place to simply restrain anyone walking on them.
In combat, if Orcus has his Wand, he is really damn hard to fight. He might have a CR or 26, but no encounter with him and his wand is ever going to be solo. As an action, he can cast Time Stop, and use those bonus rounds to summon an entire army of the dead. Seriously, he can summon 500hp worth of any kind of undead as a single action. That's two full blown Liches, an Adult Blue Dracolich, and two Crawling Claws as change. Then he can summon up to 18 Ghouls or 9 Ghasts/Wights, or 6 Mummies (dividing up the numbers by 3 for mixes), and finish off the squad with 13 Skeletons for ranged support. That is some serious firepower and action economy.
Even outside of the army, he isn’t exactly a slouch. His multiattack seems decent at first, providing melee and ranged coverage. But he can also double his damage (and some of the damage of his undead minions) by combining it with his Creeping Death Legendary action. Should he see a wounded member of the Party, he can elect to finish them off outright with Power World Kill. (With multiple Liches as backup, even Counterspell won’t stop it)
Realistically speaking, a Party's best chance against him is trying to fight him without the wand. While he can still animate a bunch of undead, he won’t have the Lich backup, or PWK. Making the fight somewhat more manageable. Either way, he is probably the strongest Demon Lord in a fight for 5e. And honestly, he is one of the few Demons that is interesting outside of fighting too.
In the Changeover, Orcus lost Chill Touch, in favor of the Necrotic Bolt attack, and gained a more versatile multiattack to boot. While he did technically lose the Magical Weapons ability, he is stated up as wielding an Artifact, so it still counts as a magical weapon. One that even works in an Antimagic Field should the PCs try such shenanigans. He also lost his madness table, but that is to be expected at this point, and quite frankly, he didn't need it to be a huge threat to begin with.
