Hi, Newbie DM here.

I am wondering what level I should ask my 3 players to be at to fight an Ancient Black Dragon. I put it in the encounter builder, and it is saying level 19 for 3 characters to fight an ancient black dragon. Does this sound right? It seems a bit high to me. I want this to be an encounter that lasts a good while, and not one where the players just steam roll my monster. I have learned not to trust the encounter builder 100%. Any advice would be appreciated.

Caveat: I have not played the high level game in 5e. Or in any edition, frankly... other than the pseudo-3e of Neverwinter Nights.

It does seem a little high level to me... I suspect level 17s or 18s could take it on without an issue. It is CR 21... theoretically that means it is an appropriate challenge for a "level 21 party" but we know the CR ratings of most monsters are inflated. At this point mentally I subtract about 20% from CR.. so a CR 10 creature, I call it "CR 8" in my head, etc. That would make CR 21 a CR 17... but I can't say for sure if the 20% rule would work on monsters over CR 8 or 10 because my current party isn't that high yet (they are only level 5, but I would have no concern throwing a CR 7 or 8 at them... just looking at the stats I am sure they could handle it).

According to the DMG, it’s a Deadly encounter for 3 19th level characters. Note that, because D&D is really, really bad if you only do one encounter a day, this assumes the characters have already taken 2 short rests and are operating at 1/3 of their resources (and thus wizards won’t outclass everyone else which would suck). THIS is the key. Never give a single encounter more than a third of the days XP or it will be steamrolled. Also note the MM intentionally rates dragon’s’ CR a bit low, so the characters losing is a very real possibility. If you want to guarantee a win (I wouldn’t, but to each his own) go level 20. Then again, it also tends to weaken monsters at higher levels, so it may even out.

That said, if you want to make sure the monster doesn’t get steamrolled, play it smart. Use the breath weapon early and often. Target spellcasters. Use the flying speed so melee characters struggle to engage. Break concentration spells. And most importantly, never, ever forget to use Legendary Resistance on the wizard’s big spell. Good luck!

Oh, and if it’s the final fight of the campaign and you’ve got a pesky healer, those three attacks are a great way to gobble a KOed character. Just saying.

D&D challenge ratings are a pretty crude tool, but PCs who know they're up against a black dragon (and thus can do things like gearing up beforehand with things like a potion of resistance) can hit well above their level. Your best bet is trying to model the fight, rather than trying to use the CR system.

You could try and playtest it yourself. Craft up 3 characters and see how they fare at level 18 and then make a decision if you should go up or down? Might eat up some time but just another option.

A big part of the challenge for a creature like a dragon is based on how intelligently it fights.  Remember its lair actions- they're highly effective act splitting up the party and keeping it off balance.  Darkness benefit's the dragon because it's got not only 120 of darkvision, it's also got 60 of blindsight, which means that it's unimpeded by magical darkness.  Also, black dragons can breath underwater.  Most PC races can't.  This not only means that if there are large bodies of water around (and there should be) it can easily break off contact with player characters at will, it also can potentially try to drag an opponent into the water with it, which is a really dangerous situation.

The difficulty of an encounter will vary based on how experienced the players and the DM are at playing their characters/monsters. It's all about the tactics. For a dragon to reach the ancient stage, they have already survived numerous other adventuring parties trying to kill them. So that dragon knows literally every trick the players might try, and it will be prepared for them. This fight won't take place in a parking lot on a sunny day, with both sides squared off toe-to-toe. 

Black dragons live in swamps. The dragon will use that difficult terrain to get the players bogged down and reduce their movement. It will have established security measures around its lair to direct the party through certain bottlenecks - there could be walls of thorns, or areas with poisonous plants, or deep pits filled with water that just look like puddles until you step there and sink down 20 feet into dark muddy water. The players will have to negotiate a veritable minefield of problems like that to even get near the dragon's lair. Check out the regional effects in a 6 mile radius around the dragon's lair: 

• The land within 6 miles of the lair takes twice as long as normal to traverse, since the plants grow thick and twisted, and the swamps are thick with reeking mud.
• Water sources within 1 mile of the lair are supernaturally fouled. Enemies of the dragon that drink such water regurgitate it within minutes.
  Fog lightly obscures the land within 6 miles of the lair.

Plus, dragons often use the other creatures around them as their eyes and ears, as an early warning system, and as defenders. Blights, fungi, assassin vines, spores, snakes, crocodiles, maybe some undead, lizardfolk, a couple of trolls... and this is just the appetizer! The party will deplete half their resources just getting to the dragon's lair - assuming they can even find it! And they won't have time for a long rest to recover because the dragon knows they're there. And once the party enters into combat with the dragon, it's got a devastating breath weapon, plus frightful presence, plus three melee attacks per round, plus 3 legendary actions per round - plus it will use its ability to fly and swim and breathe underwater to gain tactical advantages over the party. And - it's got blindsight and a passive perception of 26. You're not gonna sneak attack this guy! 

Let's look at a few quick numbers, just for fun. A 19th level fighter with a 20 Con has about 213 hit points. The dragon's breath weapon does 67 acid damage. That's almost 1/3 of your tank's health! If the dragon hits the wizard with the 12 Con, it's gonna cost that wizard almost 3/4 of their health! And that's just the first of FIVE things the dragon will do in just the first round of combat! So let's say that wizard throw Disintegrate as a 9th level spell. The dragon has +9 on Dex saves, so there's a 50% chance it'll just miss, but even on a hit it'll do about 107 hp of damage. That's less than 1/3 of the dragon's health. And melee players probably won't even be able to get near the dragon more than once every few rounds. 

TLDR - a monster is more than the stat block. It's the terrain, it's the tactics, it's the experience. Monsters know what they're doing. Especially a monster who has survived long enough to be ancient. Just imagine - the party has gotten past all the tricks, traps, and defenders, and is finally entering the long sloping tunnel into the dragon's lair. From out of the shadows they hear a sound reverberating through the chamber.... a long sniffing sound... and then a voice, as deep and malevolent as the swamp's own darkness, says...

"You smell familiar... I think I ate your great great grandfather... Would you like to know how he tasted?" 

Thanks everyone for the wonderful advice! I will take it all to heart.

i calculated it, and it seems way too low... if its the only encounter in the day, than it, according to the daily xp in the DMG, level 11... but that seems too low, and your character might die... so use the above people advise not mine maybe.

Are now pools of acid?  Is that how the story ends?

just last week I ran an adult black dragon with 4 beefed up kobolds against my party of 6 level 10s. They beat it, but one of them died.

so yeah, NOT level 12...