Well, the redo of this monster and encounter was pretty epic!!!
I used the 5.5e Adult Black dragon, and added 2 lair actions: The surge of water from nearby pools-(because I HAD to, lol), and Darkness. I also added a bite attack- so bite/rend/rend, or rend/rend/spell, or even rendx3. Wing and tail attacks were added as legendary actions. (and never used)
I also gave him thugs: 4 bugbears and a bugbear boss and 2 Dire Centipedes. To soak up attacks, and maybe deal a bit of damage.
In the end, the entire party was wounded, the Cleric nearly died, and the Druid was hurt bad. I gave the monsters a surprise round- the dragon was indeed aware of their doings in the dungeon, and prepared accordingly. Plus, we were NOT aware of the bugbears or centipedes until they attacked. Dragon opened with a breath attack nd nailled half the party. 2 saved 2 didn't.
the centipedes went after the right flank- and the thief and wizard.
The dragon and bugbears won initiative- the latter by dint of a nat 20. They mobbed the barbarian and paladin, then the Dragon charged the Cleric with a full attack. melfs acid arrow and getting crits on BOTH rends... down to 19hp. He lives ONLY because the dragon took GREAT exception to the cone of cold and disintegrate spells the wizard and sorceror threw at him- and pounced on the sorceror. or one of his mirror images, anyway, lol the druid then got him with an erupting earth. (she was quite salty about that acid to the face...AND the insect swarm)
Along the way, the surge lair action managed to only take out the bugbears, lol. WE all saved- they did not. And got dragged 20' into the pool and knocked prone.
The Druid's spell dropped the dragon to 5 hp, and since the rest of our party were engaged with the bugbears- on it's turn, it took off, retreating back to it's ledge- and dropping it's Vitreolic Sphere on us as an EFF YOU!!!! it caught the whole party. AGAIN, Cleric was dropped nearly to 0 (thank god for the Aura of Vitality he cast on the way in- it saved his life!)
then came my favorite part: the wizard said, huh huh NO- screw YOU!! and sent an upcast 4th level magic missile at him. Essentially blowing it out of the air.
As an added bonus, the thief used sneak attack on the last bugbear- who was just climbing out of the water going after the Fighter. Natural. Effing. 20. That's 14d6 on an arrow from a shortbow. He had 7 hp. The amount of damage was ungodly overkill, lol. So, for comic effect, I said the arrow caught him just getting up- it went in his nostril- and came out his butthole.
Surprisingly, the thief declined trying to find and salvage that arrow.
The centipedes are ice sculptures- they got in the way of that cone of cold...
We survived- and have a VERY nice dragon hoard to claim, but I have a feeling the divine casters will be out of slots and lay on hands before we leave this cavern. The intent is to gather our caches of loot and use the Cleric's Amulet of Teleportation to return to the gate we arrived at in the foothills. via Teleportation Circle. then use that gate to bring the Wondrous Lanthorn to Elminster's tower- completing the quest.
All told, between this and the last adventure, they each now have enough $$ to turn their houses in Daggerdale into whatever they were planning. (a tower for the wizard, a school for the Paladin, a shrine to Selune for the cleric, etc.) plus various nice items to furnish them as mementos. The barbarian doesn't much care- he has his axe and a folding boat to go explore the wilderness. Maybe find a lady barbarian to chase after in Rashemen, where he is from. Until, of course, wanderlust and the urge to adventure takes us again. (or we run out of money, lol)