Is a young black dragon enough of a challenge alone for group of five level-5 characters?

Question

I have a 5-member party in D&D 5e (ranger, fighter, sorcerer, rogue and bard) who are all level 5. They will soon be meeting with the "big bad", a young black dragon, in his lair which contains pools of acid. They would most likely be fully rested before the fight.

I know the CR is 7, but I was wondering whether it would prove to be a satisfying battle, in the sense that they may come close to defeat and might be able to beat the odds.

Is this fight well-balanced? Would it be challenging?

If not, should I add some hatchlings or small mobs to adjust the action economy appropriately?

Answer 1

This is likely between Medium and Hard difficulty encounter for the party

You can find general guidance on encounter difficulty in the Dungeon Master's Guide (DMG) in the Creating Encounters Section. For level 5 characters, the XP table shows:

Level ------- Easy --- Medium --- Hard --- Deadly

[...]

5th ---------- 250 ---- 500 -------- 750 ----- 1,100

This means that 5 level five characters would treat a Young Black Dragon (XP = 2,900) as a Medium encounter (5 * 500 = 2,500). While there are some monsters that break this mold and difficulty can be increased depending on the composition of enemies, this is a good rough metric.

Consider the level of optimization in your party

If your characters are particularly good at dealing lots of damage or protecting a single character, boss battles like this will be easier than usual and you should increase the difficulty by about 100 XP per level so 500 XP (this is a number I've used in the past and has worked wonders for me).

If you have new players and/or players who are not tactically minded or are unfamiliar with the mechanics, you should decrease the difficulty to the next lowest setting (an Easy encounter will be Medium, a Medium encounter will be Hard, etc.)

How hard do you want it to be?

satisfying battle in the sense that they coming close to defeat and beating the odds.

This sounds like you want it to be somewhere between Hard and Deadly, which a dragon on its own won't accomplish (unless your party fits the "new player" category I described earlier).

To fix this, you will need your XP approximation to equal 4000-5000. Adding just one Black Dragon Wyrmling pushes the number to the top end of this value at 5,025 due to the Modify Total XP for Multiple Monsters section in the DMG (2 monsters means a 1.5x multiplier).

For a lesser difficulty, but still a suitable challenge, try adding a CR 1 monster like a Goblin Boss or just add a couple of terrain effects to increase the difficulty indirectly.

For a higher difficulty, pushing into the Deadly range, you will need a CR 3 monster like a Bugbear Chief, or add terrain effects or traps to the Dragon and Wyrmling encounter.

Answer 2

No, they would likely not break a sweat defeating a Young Black Dragon.

The dragon, at CR 7, is worth 2900 experience points.

Checking the DMG on encounter building (pg 81) we can calculate that a 2900xp encounter for a group of 5 characters at 5th level would be classified as "Hard", but only barely (as anything between 2501xp and 3750xp is Hard). Note that "Hard" is described as:

Hard. A hard encounter could go badly for the adventurers. Weaker characters might get taken out of the fight, and there's a slim chance that one or more characters might die.

Not really a "beating the odds" kind of fight. Especially a party that is fully rested will likely have no problems at all defeating such a creature. Perhaps if they were already worn out from an entire day of adventuring, they might consider this scary. But if they know this is the biggest, baddest thing in the dungeon, they will just drop their powerful abilities on it and kill it easily.

You should probably add in a bunch of other threats to up the scale, to at least the "Deadly" category. The guide can instruct you further on how to do it, I'd suggest reading that section.