Ugh, I know. Swarm mechanics kind of trivialize a lot of BBEG fights. They have been clamoring to fight a dragon since level 4 - but a good breath attack from an adult could be a near instant TPK.
That said, an adult could simply be a cliffhanger for another day - flyover with a breath...
The party I DM consists of 7 level 5 players (Barbarian, Warlock, 2 Rogues, Ranger (dragon is his favored enemy), Sword+Board Warrior, and a Druid of the Moon).
This is a magic item friendly campaign - so I think they might be ready to take on a Young Black Dragon.
If I simply plop the...