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?