This was part of my longest D&D campaign. It was pretty homebrewed 3.5, and we had the perfect mix of personalities. They also seemed to really appreciate all the work I put into writing for them.
The one thing they all hated, was the fact that it's nearly impossible for me to write without hiding puns in the content. It's compulsive. Even in my English classes, I would pepper in sentences that contained the most absurd puns.
It's a problem I've no intention of correcting.
So, in this game, the players have gotten to level 8 or 9, and they were exploring a new city. There were tons of things for the characters to do, NPCs to talk with, and thousands of little threads for them to pull on, but the one they went after most was this mysterious, humongous library.
Unbeknownst to the players, my DM notebook contained the helpful reminder "something something mad god's library" and the stupid punchline. That's it.
So, flying by the seat of my pants, I came up with this crazy layout. Eventually I sat down and wrote out a huge backstory for it, but it was still with the one gag in mind.
All the shelves were made of faded books, and all the readable books were in the wrong places. Any time they wanted to find something, it was inevitably in a different section: a famous NPC's history in the home and garden section, research on dragons in the fantasy section, poisonous alchemy in the cooking section, ect. This confused the shit out of them, and almost every second of down time they got was devoted to exploring this weird library.
There was a tall, creepy librarian (often the only other person in there) with a graying hair-bun and a hooked nose. She wore a black gown, and seemed to glide everywhere she went. She hardly spoke to them, but would point as a means to answer questions, and shush them if they got too loud.
Eventually the players found a back room hidden behind a bookshelf. It had a "Members Only" sign on it, and when they asked the librarian about it, she quietly handed them a long form to fill out. (There was no trick here, but my players know better than to trust something that seems easy.)
Rather than get a library card, they resort to stealing books and replacing them after use. They soon noticed that if they left a book out of place, it would inevitably wind up in its randomly specific location. This weirded the rogue out most of all, and he convinced the fighter and the cleric that their best option was to break into the members only area in the middle of the day. Naturally, the rest of the party followed along.
When they got to the other side they found this impossibly gigantic labyrinth of bookshelves. As they walked around, they noticed that the shelves were moving and changing on their own and they had no idea where to go. Well, two of them could fly (cleric and the fighter) and one was a teleporter (rogue,) so they scouted above while the others (witch and samurai) walked below
The rogue immediately noticed that the librarian was coming after them; shelves moved to make a path, and she seemed to glide straight to the people on the ground.
Well, never one to let a good idea come between him and an irrational impulse, the cleric took off in a dive straight for her, and brandished his warhammer with a natural 20 toward intimidation. The terrified librarian shrieked and collapsed onto the ground, pulling books down on top of her and spasming in shock.
Ever the honorable helper, the samurai leapt into action; pulling a syringe from his jacket. (Ok, so about that syringe... It fully heals and negates all status effects... But it also sends you into an uncontrollable rage) The samurai plunges the "health potion" straight into her heart, and fully injects the serum.
The table went silent, apart from my laughter, and the fighter said "I can not believe you just did that."
Before they could really reflect on the choices made, the librarian let out a howling screech that knocked books off the shelves, and shook the floor. As the samurai backed up, he saw the librarian's torso crack and push outward. Still screaming, her face began to elongate, and her hands stretched into massive claws. Her back arched, and as the skin was stretched to its limit, wings shredded through where her shoulder blades had been.
At this point everyone was spending so much time arguing in character about what to do, no one did anything. The woman's face was shredded as a mouth in the shape of a humpback whale pushed out; its baleen resembled the fanned pages of a paperback. As the monster grew, it started standing up on all fours. Huge cracks ran the length of the floor where it stepped. The librarian's skin was shredded like a bad sunburn to make room for leathery scales. Along its joints, and down its spine were golden bookbindings. Its huge wings fanned out to reveal feathered pages of arcane script, scrawled haphazardly.
The witch was the first to act. She ran up to the now 15 foot monster, and was backhanded into the bookshelf the rogue was standing on. In a single round, the monster had grown to over 30 feet tall, its weight started putting a huge strain on the ground. In the next round, the floor gave way and everyone who was not flying began to tumble into darkness.
Thinking quickly, the rogue reached out with his ability and saw that the floor was almost eight hundred feet down. He began teleporting the non-flyers to safety while the cleric and the fighter used their illuminated weapons to battle the slow-falling giant; it's enormous, leather bound claws, raking and clasping for them.
Dodging debris, and fighting a leathery whale-dragon, they eventually got within range of the witch, who proceeded to send torrents of flame up at the monster (and them.) The cleric used his warhammer to cripple its wings, and send it plummeting. When it landed, it roared up just as the fighter came down, rolling a perfect 20 and sailing bastard-sword-first into the creature's mouth, and out of its throat.
Pages began to burn away as the monster crumpled in a heap. Bleeding and weary, our adventurers found themselves in a massive underground cathedral. Deep, slow laughter came from the darkness all around them.
"So," said the god of madness, "I see you've defeated my bookwyrm."
Silence.
The rogue started laughing and shaking his head. The witch looked incredulous. The cleric put his head on the table. The samurai just smiled and tried to tell if I was serious. The fighter pushed his chair back and said "I'm going to smoke. You are the fucking worst."
As soon as they heard it, they knew that the past two months of their gaming lives had been an elaborate set up to the dumbest shit I could think of. I honestly would be hard pressed to find a moment where I made myself laugh harder. Just, the sheer amount of time and planning it took for that payoff was so worth it. I had tears streaming down my face as they left my house. The beautiful stupidity of it makes me smile to this day.
We took a week off and picked up from where we stopped. The game went on for another year and a half.

source: https://www.reddit.com/r/DMDadJokes/comments/513py3/longi_once_spent_two_and_a_half_months_setting_up/