Blue
Ravenous Bugblatter Beast of Traal
That was just me gushing about 13th Age as a system. And if you are looking grimy, it may not be what you want - 13th Age tends heroic, just like D&D.@Blue much of that does sound interesting for certain kinds of campaigns but I am not getting a grimy dungeon crawl vibe from your description.
To talk about the specific example, in the Dragon Empire (the default setting), there are Living Dungeons. They burrow up from deep underground, and are malignant and have a semblance of life - and more.
Eyes of the Stone Thief is a award winning campaign/multi-adventure by Gareth Hanrahan around one particularly deviant and crafty Living Dungeon. It can be run as a campaign, or broken up in parts and integrated into and around an existing campaign.
If you want a taste, there is an offical 5e conversion of 2 level on DrvieThruRPG. Pay what you want, recommendation $2.95. I have the original for 13th Age, so I haven't checked out the 5e conversion.

Eyes of the Stone Thief - 5e Compatible - Pelgrane Press | 13th Age | DriveThruRPG.com
Eyes of the Stone Thief - 5e Compatible - Can you kill the dungeon before it kills you? In 13th Age, living dungeons slither up through the underworld and invade
I suggest you check out a few of the reviews. We get things like:
"It is truly a great campaign, and is now the standard against which I measure megadungeons".
13th Age is already D&D with the volume turned up to eleven, and I can picture Ryder-Hanrahan grinning maniacally as he twists the dial even further clockwise. Eyes of the Stone Thief is completely mad, and my main concern from running the game is how I am going to herd the players back into a gritty, low-magic game afterwards.
It is truly a great campaign, and is now the standard against which I measure megadungeons; but the GM needs to put a lot of work in, both before it begins and during play."

Review: Eyes of the Stone Thief
“That was nuts!” – Dag, Angry Beavers As you may have guessed by now, this is the backbone of the Hearts of Stone campaign, whose session writeups you see on Wednesdays. It’…
I'm really not into megadungeons, or dungeons in general. The classic dungeon setup of a room full of random traps and monsters never made much sense to me, and seemed kind of dull. I also don't usually find reading adventure modules that interesting.
But when this book got here a few days ago I was intrigued, mostly by its sheer size. I mean, it's a full-size 300+ hardcover book. I was like, "What kind of module IS this?" So I started reading.
And, wow.
Calling the Stone Thief a 'dungeon' doesn't really do it justice. It's a crazy brilliant plot hook + a strange and often lethal environment to explore + a giant puzzle box + a monster all rolled into one. The Stone Thief is an enormous entity or magical beast that consumes castles and temples and even cities to cover itself and grow in strength. The only way to kill it is to descend into its belly again and again and again. And it is different each time.
There are dungeon-like areas, but there are also numerous factions and strange creatures and environments within the Stone Thief. It's superbly imagined. The book gives you the pieces to craft an ever changing and ever-evolving dungeon to really challenge the players. Oh, and it's not afraid to throw really hard and even likely lethal stuff at them, although it avoids "gotcha!" traps.
Eyes of the Stone Thief: this ain't your grandpa's megadungeon
To be honest, I ordered "Eyes of the Stone Thief" mostly because it was 13th Age, and I usually enjoy books put out for that, and because I'd seen some people around here speak highly of it. I'm really not into megadungeons, or dungeons in general. The classic dungeon setup of a room full of...