Asisreo
Patron Badass
To give background, the dragon is too lazy to have actually set anything up. The acidic lakes were there before he was, he just decided it was the best place to settle down (acid immunity and puddings don't bother him any).You can follow the KISS principle and still have a reasonably challenging encounter that prevents the party from waltzing in and going nova. The main thing to think about is designing the lair around the monster such that if the party tries to just sit there and mindlessly dish out damage, they're all going to die. You don't need tons of zones and ability-neutralizing things. Here's how I'd do an adult dragon (whom we've given acid immunity):
Quick sketch of the above-ground part of the lair (embellish to taste):
View attachment 133880
The pools are connected by an underground network of passages. They additionally have a layer of mist above them, allowing the dragon to stay hidden as he peeks above the surface to surveil the party. The cave is dark, giving the party -5 to passive perception. The dragon's main tactic is to Hide, peek above the surface of a pool, use his breath attack on whichever branch he figures will hurt the party the worst, and dive back down beneath the surface, where he waits for his breath to recharge. Following the rules for hidden combat, Readied actions are resolved after applying damage except in the event the dragon fails his Hide. Wizards who Ready spells may, therefore find their concentration broken by the breath weapon.
Once the dragon is below half health, he retreats to his main cave, where he quaffs a couple healing potions from his hoard, hides somewhere, and waits for the party. If the party retreats, he vacates the premises, taking the best treasure with him but leaving behind some consolation prizes.
You can see here if the party uses low-level tactics, they will get their asses handed to them. I'm sure people used to playing high-level casters have all sorts of awesome stuff they can do to win this, and that's the point. Make them do their awesome stuff instead of just spewing out Disintegrate a bunch.
The antimagic and wildmagic zones are always more frequent the closer they are to the inner planes. They aren't going to be placed particularly well, but the dragon does put his treasure in these zones to prevent them from being stolen.
The dragon doesn't expect intruders so he's not particularly alert but he's not aloof. If they get too close, things could go wrong.
But ultimately, the dragon's tactics will be about diving in and out of the acid and perhaps dragon characters down alongside them. 10d10 is considered "dangerous" damage at this level but its definitely significant enough to do serious damage.
Oh, I forgot to mention its a DC 16 con save for half-damage so its not like a guaranteed 10d10 every round: certain characters might have the proficiencies to mitigate and the armor basically reduces the damage from 55 all the way down to 13 per round which isn't horrible.
But that's what I think is very cool about it. If the party keeps their eyes up, they should be able to get through it quite easily. The difference is that unlike Disintegrate spam, the players had to use tools they both earned and built skillfully.
That's my favorite part of high-level D&D and its where I derive the most pleasure: when they say "Woo! That was awesome! I can't believe we beat it!" Even when they never dropped below half-HP.