First off, if you know the significance of the Black Eye gang, don’t read any further. (Spoilers!)

Hello, all. The current arc for the campaign I’m running is going well so far, with lots of intrigue and infiltrations in a wealthy city environment. Next session, the PCs are going to confront the ruler of the city for the final battle of the arc, and I have most of the fight planned.

The ruler is just a normal noble, though, and just fighting through his minions while he sits in the background seems like it would be kind of underwhelming. Any ideas for how I can make him be a threatening presence in the fight, rather than a bag of HP to take out after the real fight?

Give the Noble a magic chair that can basically casts Tiny Hut on the person inside instantaneously, but also has a shorter duration... that way it's just a magic item doing it for him, but it's something so big and cumbersome that the players can't just pick it up and keep it for themselves. Then give the Noble a Necklace of Fireballs... spells can't pass out from a Tiny Hut, but objects can be thrown through... a Necklace of Fireballs doesn't actually cast the spells, it just activates as the spell when it hits the ground. So he's in the background, virtually invulnerable, and he has a method of dealing a lot of damage without needing to include some detail like, "Oh, by the way, he's also a world-class archer".

You could have the noble utilize traps, mundane items, or magic items. He could pull a lever as an an action that releases a volley of arrows onto the players. He could be hiding a ballista back near his throne that he tries to shoot the players with, albeit likely with disadvantage if he doesn't know what he is doing. Like what TransmorpherDDS suggested, magic items of a varied capacity could be used. As a noble, he'd likely have the financial means to procure some pretty useful or powerful items. Just be mindful, depending on what level your players are, if they win and loot the noble, they'll be taking that magic item most likely, so don't make it too powerful unless you're ready for your players to have that now too. 

A way to add another element entirely is to have the noble escape. Maybe under his throne there is a secret staircase. If capturing or killing the noble is vital to the player's plans, it will likely cause at least one to shift their focus off of combat to pursue the noble so they don't get away. 

Lastly, depending on the noble's personality, there is the likelihood that when backed into a corner, they will be pleading and bargaining with all they've got to be spared. Another dilemma for your players to face.  

Quote from Lyrak >>

 Just be mindful, depending on what level your players are, if they win and loot the noble, they'll be taking that magic item most likely, so don't make it too powerful unless you're ready for your players to have that now too. 

 

 That's one of the biggest challenges I face as a DM... I love looking up magic items and getting ideas for how to use them... but I know if anybody has a magic item of any kind the players are eventually going to end up with it. 

Although loading up the noble with magic items is a good way of making him into a sort of "pinata" that pays off after the fight.

Leave the noble as a creampuff.

Then, at the climax when all their minions are dead and gone have them surrender and ask for mercy. Players get all kinds of discombobulated when they have to think about not using the sword as the answer. At that point killing isn't... good

Give the noble a scroll with polymorph. When he uses it, as his last ditch gasp, the players can't get it, so no worries about giving up a superior magic item. Now the noble isn't A bag of HP, he is whatever you want him to be. This will really turn it up a notch. Let the Real Fun Begin.

You could give the Noble a mount to join the combat.  Depending on the party level, it could be something like a dire bear or a wyvern.  As such, it's not so much his abilities, it's his significantly powerful pet the party has to deal with.

Quote from Agile_DM >>

Leave the noble as a creampuff.

Then, at the climax when all their minions are dead and gone have them surrender and ask for mercy. Players get all kinds of discombobulated when they have to think about not using the sword as the answer. At that point killing isn't... good

 Leave the noble as a creampuff, have him start pleading, and through backstabbery reveal some hidden, stronger form that tries to kill all the players in its madness.

Not all "normal nobles" have to be completely ineffective in combat. It's conceivable that he might be a retired knight, a champion duelist, etc. If that makes sense for the noble, then you certainly not just make him a total pushover without compromising the "normal noble" story.

Alternatively, you could give him some support abilities- there are several different creatures that have actions/bonus actions/reactions that they can activate to benefit other creatures (such as the knight's Leadership ability). You can modify and reflavor an ability like that, or adjust it for your purposes, and that way even though he doesn't have to be a particularly good fighter, he can inspire his troops. Depending on how he motivates or controls his followers, then you have a lot of options:

A noble who is magically compelling his guards could use a command word to activate an item to take control of them and use their reaction to attack.

If his guards are more afraid of him than they are of you, he could use an intimidating shout to threaten them into fighting with more fervor, giving at least one of them advantage on their next attack as they are desperate to avoid his wrath.

If gold motivates his troops, he could offer a bonus to the first guard to defeat a member of the party, inspiring them with a bonus to their attack rolls.

If he's a sound tactician, perhaps he has a reaction he can use to encourage his guards to move (such as when one of his allies falls, he can use a reaction to order another to fill the line) or act (give a guard the benefit of the dodge action using his reaction by pointing out the party's barbarian getting ready to destroy him).

So on, so forth. These options allow you to stress that he, himself, is not great in combat, but he is a threat in so far as he can make the other combatants more difficult to dispatch. Once that's done, you can play his personality. Perhaps he, once his fodder is defeated, will try to do it himself and defeat the party. Maybe he'll grovel, asking for mercy now that he doesn't have any defenses. Whatever the choice, you can leverage what the party already knows to give him some abilities suitable for his personality that might make the fight more interesting, without needing to make him either a powerful fighter or completely worthless bag of health (though, if you're using the noble statblock, he will not be *much* of a bag of health).

Quote from CadenzaTheAdventurer >>

Give the noble a scroll with polymorph. When he uses it, as his last ditch gasp, the players can't get it, so no worries about giving up a superior magic item. Now the noble isn't A bag of HP, he is whatever you want him to be. This will really turn it up a notch. Let the Real Fun Begin.

 A noble can't use a scroll. You can only cast a scroll if the spell is on your class spell list, and without any levels in a caster class, said noble has no class spell list, and therefore can't cast a spell off a scroll. Of course, NPCs don't have to follow the same rules as PCs, so you could always use that as our reason why this particular noble can use a scroll.

Or, you might give him a ring of spell storing, with polymorph loaded in it. Maybe also a potion of invisibility. When things get sour, drink the potion, then use the ring to change into a bird or something and fly away instead of fighting.

The noble doesn’t have to be an active combatant to be dangerous. Look at the examples from other sources where the BBE was/is a waif, but their intellect, influence, and resources are what make them dangerous.

Take for example, one Professor Moriarty. Everyone knows he was Sherlock Holmes archenemies. Right? in Sir Arthur Conan Doyle’s original works, he only actually ever appeared in one story, the one where he and Holmes both died. He was actually only ever created for the express purpose of killing off Holmes for Doyle. That’s it. He was the BBE in another Holmes story, but never actually showed up in person. Everything else Doyle ever wrote about him at all was just Holmes mentioning him. Professor Moriarty is widely believed to be one of the the most iconic villains ever created, and he was hardly in the original works. Wanna talk about “threatening?” How effing threatening is that!?! He was barely ever a character, and yet he is held as the gold gorram standard of “evil genius BBEs” ever since?

Someone the heroes can see, touch, and fight… 🤷‍♂️ that just like everything else they can already interact with. Someone they only know of as a name whispered in the shadows who can throw a city at at them without ever making an appearance… the villain that is (as Holmes called Moriarty) “half of all that is evil” in the city, but whom they can never confront….

You question made me think of game of thrones what I remember of it two "nobles" were very threatening due to their sadistic antics and wealth... so hirelings and human shields... otherwise some last ditch infernal barging would spring to mind   

 Definitely make use of underlings.  In one of my previous games, I had a lot of fun running encounters using Merregon as bodyguards to take hits meant for their charge, a human who apart from having a revolver and a couple of magic scrolls was entirely unremarkable himself; it ended up being one of the best combat encounters from that campaign.

Quote from TransmorpherDDS >>

Give the Noble a magic chair that can basically casts Tiny Hut on the person inside instantaneously, but also has a shorter duration... that way it's just a magic item doing it for him, but it's something so big and cumbersome that the players can't just pick it up and keep it for themselves. Then give the Noble a Necklace of Fireballs... spells can't pass out from a Tiny Hut, but objects can be thrown through... a Necklace of Fireballs doesn't actually cast the spells, it just activates as the spell when it hits the ground. So he's in the background, virtually invulnerable, and he has a method of dealing a lot of damage without needing to include some detail like, "Oh, by the way, he's also a world-class archer".

 This is an excellent suggestion and what I'd go with.

For a noble to be threatening, they need to threaten more than just the heroes, and they need to be active in the background - you can't have them sitting perfectly still in the last room of a dungeon waiting to get the snot kicked out of them.

Let's say the party is a paladin, a rogue, and a wizard, who were asked by the town elders to solve a crime for a thing which was stolen. They fight their way to the BBEG, this noble, and they are confronted with a programmed illusion of them which explains how, as they have been fighting through the sewers to find them, a witness has come forward and sworn to the elders that they saw the Rogue hiding the thing in their room at the tavern. The rogues rooms have been searched and the thing was found there, along with the outfit which matched the one the guards saw, so now the party is wanted for crimes against the crown. Then the BBEG might say that they are welcome to come and try to chase after them, but then who will save all those poor souls trapped in the orphanage - oh, didn't I mention? That old, rickety wizards tower - yes, the one I gather you were so keen on searching for new spells - in these high winds, it would be a miracle if it didn't come crashing down onto that orphanage. And books are so flammable, I daresay that it would be quite the inferno. What say you, Paladin? Do you believe in Miracles? I can offer you one; leave me be, and that tower, and those orphans, might just be saved.

Then if they decide to chase them, boom, half a wizards tower crashes through the roof ahead, sealing them off from the BBEG to let him get away.