Hey Fellow DMs,

I have a campaign in development, and I am going to have an evil king/dictator type of thing that the party has to dethrone and take down. Just wondering if anybody has any cool but INCREDIBLY powerful character builds that I could use for him. The party will consist of 2 members most days, but on days where the rest of the party actually show up, it increases to 6 members. I have not paid for any content. 

Also I am a new DM, so does anybody here have any tips for me about campaign creation and management? 

Thankyou

It's quite difficult to match the power level of 'incredibly powerful' without having some clue as to the level of the PC's. 

Also, trying to build incredibly powerful NPC's is kinda dangerous. The key to good encounter design isn't to set out to make a very powerful opponent - but to design an interesting fight. For instance, I once designed a fairly underwhelming critter called a maggotaur. Basically a large larva with a sorta upright upper body with a lamprey-like maw full of scary teeth.

They had their own small surprise - they were dispatched quite easily, but once they got their maw attached to some hapless victim, they gained in health and stats, and their disgusting white skin grew .. reddish. 

Now, if one of these critters got to munch on enough blood and guts, it would transform - or rather grow - into the Maggotaurus Rex. A far more threatening and powerful creature, all shiny red and full of blood wonderful vitality. 

I'm not saying this is the gold standard for encounter design,  but it's an example of a fairly harmless - but disgusting and annoying - critter that suddenly became an almost overwhelming foe. It was as close a thing as one might hope for, all PC's at low HP and out of ressources when it died. It had a little arch to it, it scaled up nicely, there was a story background (there was an undead ghost-like lady-creature that 'gave birth' to these things). 

Of course, the ghost-lady would be the actual BBEG in my campaign, but that encounter was much more difficult to describe. It was a circus (literally), there were mirrors and drapes and multiple levels. 

Anyways, the point is that you shouldn't, perhaps, aim for the NPC being dangerous - but for the encounter to be fun and exciting. And those two aren't automatically the same.

I would gently nudge you to reconsider treating NPCs as PCs with class levels, and instead suggest building the NPC as a monster stat block. For one thing, it's easier to balance monster stat blocks. It's also easier for a DM to work out how to make the NPC effective in combat.

That aside, do you have any details about the NPC that you'd like to incorporate? Special abilities and traits, perhaps? Also, at what level do you intend for the PCs to confront the NPC?

Gnomarchy is right. In general, it's best to steer away from using class levels when creating a villain. Even if you make them level 20, it's still one person versus many - the superior numbers will prevail simply because they can attack more often to whittle down hit points. D&D was designed to get around this by having monsters use their own rules: they have more HP than a character would, hit harder than a character could, and/or they hit more times per round than a character could. So for your archvillain, a monster stat block is the way to go.

To get around the party ganging up on the villain, you'll want them to have multiattack or Legendary Actions, the latter of which lets a baddie move or act against the party when it isn't their turn. Look at monsters like the ancient brass dragon, vampire, kraken, mummy lord, lich and aboleth for inspiration. 

In addition to hitting more, you want your villain to be tougher to beat. Consider giving them advantage on saves against magic (a trait some monsters have called Magic Resistance) and/or Legendary Resistance, which lets a creature auto-succeed when they would otherwise fail a save. Legendary Resistance prevents an anticlimactic battle - you don't want your super skilled evil mage to be taken out in the first round because they somehow rolled an 11 against Dominate Person. Most legendary creatures have 1-3 Legendary Resistances, but if your party will be very high level and have a ton of magic items, having up to 5 legendary resists isn't unheard of.

Finally, your BBEG is unlikely to be fighting alone. They'll have minions and lair effects. Think about the villain's powers and influence in society. Are there fiends or cultists who will protect them? Does the villain have innate power over the elements or their home base? A climactic boss fight is often more interesting, and more challenging, if the party has to overcome environmental dangers (like mini earthquakes or blinding lights at the top of a round) and evil guardians as they're trying to defeat the villain. This also adds to the amount the villain's side can attack per round, helping to create a balanced and challenging fight - but it also means the archvillain itself doesn't necessarily need to be super powerful.

Regarding sources, all of the monsters I listed are available in the basic rules and you should be able to see them. DMs Guild and DM Dave also have a lot of free content that might help you as you create monsters and your villain. Finally, Matt Colville has a fantastic video on action-oriented monsters that might help you, too.

I don't think the original post says the villain is built like a player character?

The other advice is sound, though I think it's missing the fact that there are only two players (what makes solo bosses relatively weak a lot of the time is that players can just do more things each round, but fewer players means that's less of an issue) and the fact that the poster has not bought any content, which means they don't even have the guidelines for creating a monster/NPC laid out in the Dungeon Master's Guide.

What you want to do, then, is estimate what level the players are going to be. Go to the encounter builder tool on D&D Beyond. It can be a bit misleading, but it'll let you filter down to a list of monsters in the basic rules (thus accessible for free) and figure out what CR of monster will be a hard or deadly encounter for your players on paper. If you're really committed to a solo enemy, filter down to that CR. But consider playing around with a slightly weaker boss with a couple "henchman" monsters backing him up, since that will be a tougher fight. In any case, this is all just to find a target challenge rating.

From there, you want to look for (ideally) legendary actions, or at least multiattack like the others said. Those features are good at making an encounter more dangerous. Now, maybe what you find isn't a very close match for what you imagine the villain's capabilities to be, but maybe you can modify it. It might at least be a good start for things like health, armor class, attack bonus, and damage, and you can fiddle with the specifics, maybe hunt down other monsters/NPCs with more on-flavor features to replace any you don't like and so forth.

I will suggest don't build the stat block (not a character sheet, they are not a PC) until just before the Players get there and there will be a fight.

Quote from AEDorsay >>

I will suggest don't build the stat block (not a character sheet, they are not a PC) until just before the Players get there and there will be a fight.

 Ok

So, a bit more clarity...

As noted above, Arch-Villains are essentially Monsters in the way 5e works. They use Stat Blocks, like any other monster. For that matter, if it isn't a Player's Character, it should use a stat block (and have a CR and all that). basic reason for this: Arch Villains aren't limited by the classes on wha tthey can and cannot do. THey don't have a "class", they have a "how hard is this thing going to be to beat" rating.

This means that when you do create them, you can essentially give them the abilities that you think they will need in a given circumstance. Ergo, you really don't wantt o create their stat block until you get much closer to the point where the conflict will happen, because you can then decide to make them take advantage of weaknesses or strengths of the PCs *as they are the time*. Other wsie, you are guessing and if you opt to power someone up or they choose a path you didn't think of, well, now you can more effectively tailor the bad guy to fit your needs.

THere is another reason: narrative impact. if they are an Arch-Villain, they have Villains and Lieutenants and underlings and are always at the far back of everything -- untouchable, out of reach, watching. THey are scarier if you don't have a stat block for them. More flexible, too; "I hit him with a magic missile!", well, bish, now he gonna remember you, and he knows the Players can hit him at distance so he's further back.

IT also means that if you want him to be untouchable until the final fight, he is. For the most simple reason: he has whatever stats work in the moment.

Also, when you do this, you can let the bad guy's powers grow out of the adventures that lead up to the final face off.