I've been running the adventures in Candlekeep Mysteries lightly reskinned for my Krevborna setting. The characters are all employed as members of Creedhall University Library's "Special Collections" department, aka adventurers. This is a recap of what happened in "Shemshime's Bedtime Rhyme." Fair warning: spoilers lurk below.
Rufus Clarke, human barbarian played by Steve
Doctor Tobias Wolfe, human sorcerer, played by Dennis
Rufus and Tobias were assigned to help in the restoration of the Firefly Cellar, an old underground library archive famously lit by fireflies kept in lanterns and jars down in the depths. The duo descended into the Firefly Cellar via a book lift; they first encountered an elven scribe named Varnyr who asked them to take a load of books into the level below to a man named Ebder.
Ebder, however, didn't have a use for the books, so he sent them even further down in search of K'Tulah, a tabaxi scholar currently researching folk magic. They also noticed that the ceiling of this floor featured a ridiculous statue of an open book suspended from chains. The words "BE CURIOUS" were etched on its front cover. After poking around in the lowest floor, they startled K'Tulah by abruptly bursting into her bedroom. Once things were smoothed over by the delivery of the books, K'Tulah showed Rufus and Tobias to their chambers for the night.
In the morning, both Tobias and Rufus awoke with a song stuck in their heads. The song didn't seem familiar, but the melody was insistent. At breakfast with the assembled group working in the Firefly Cellar (Varnyr, Ebder, Ebder's daughter Gailby, K'Tulah, and Crinkle the kenku caretaker), everyone began to hum the song. In fact, it was only with force of will that anyone could avoid humming the tune long enough to speak! Everyone was perplexed by this phenomena; no one recalled hearing the song before, but humming it was irresistible.
Varnyr got up from the breakfast table and made her way upstairs. Everyone could hear the book lift ascend, followed by the sounds of the hatch leading down into the cellar being locked multiple times in a myriad of ways. Varnyr explained that when she had first arrived at the library over six hundred years ago the librarians were concerned about an outbreak of "singing sickness." Because the sickness was communicable, Varnyr had alerted the librarians above about what was happening in the Firefly Cellar and had sealed off access to the wider world. Everyone was now trapped below, hoping that those above could discover a cure. K'Tulah did not take this well and began to panic. When asked what could be done, Varnyr gestured to the library around them and stated that perhaps a clue to a cure was somewhere amongst the mess.
A high-pitched wail broke the silence. Tobias and Rufus ran to find the cause and discovered Ebder on the ground, his eyes bulging as he screamed. His daughter ran to him, with Crinkle trying to comfort her. As the duo examined him, the bookshelves in the room began to divest themselves of tomes in a barrage that battered everyone in the chamber. When the assault had subsided, Ebder stopped screaming, though now the song began to alter. Instead of just a hummable tune, the song began to have lyrics and everyone in the Firefly Cellar began to sing. The song seemed to be a nursery rhyme about the doom that befell a family: a wife who slays her husband with a scythe, a dog that bites off the mother's hand, a son who drowns...all hinted to be caused by a supernatural being called Shemshime.
Rufus and Tobias noticed that a voice not belonging to anyone present had joined them in singing Shemshime's song. They followed to voice to its source: a secret alcove in Crinkle's room. Inside the alcove, they found a collection of items that Crinkle had stolen from the others and a strange mechanical pop-up book whose pages illustrated the tale told by the song. The book's spine was also a music box, though it was obviously damaged.
Calling a meeting of all within the cellar resulted in Ebder and Crinkle coming to blows, as one of the things Crinkle had stolen was an amber ring that belonged to Ebder's deceased wife. A calm emotions spell from Tobias broke up the fray. K'Tulah lost her grip on sanity, fled to the first floor of the cellar, and began to use her cat-like claws to climb up to the sealed hatch. Her attempt was in vain; she could not open the hatch, she lost her balance, and plummeted to her death on floor below.
After the remainder of the group reassembled, another death occurred. Seemingly in a daze, young Gailby picked up the mechanical book and brained Crinkle to death with it. Varnyr picked up a small knife used to repair books and began to advance on Tobias and Rufus, but Tobias cast a charm person spell that brought her to her senses before she could act under Shemshime's influence. Ebder agreed to let Tobias put them into a magical sleep so they couldn't be taken over by Shemshime. As an elf, Varnyr was immune but she agreed to lock herself in her chambers while Tobias and Rufus figured out what to do.
Tobias decided that the best course of action was to attempt to repair the music box on the book's spine. However, as he fiddled with the book, it attacked his mind with psychic force. Although repairing the book brought him within the reach of death, Tobias managed to fix it. Doing so revealed that there was more to the story: another page turned and the pair were treated to the image of a young woman tricking a shadowy form into standing beneath a millstone, which she then cause to be dropped on top of him. The song they were singing also added a new verse that told this part of the tale.
The music box began to leak black steam that coalesced into a human-like form: Shemshime was here. Wasting no time, Rufus flew into a monstrous rage, climbed onto the granite table, leapt into the air, and grabbed onto the stone statue of a book. He pulled it from its chains and sent it crashing down on Shemshime. An explosion rocked the chamber as the statue shattered...but Shemshime was vanquished. Everyone could finally stop singing.
In a few days time, the Special Collections Department sent a squad of heavily armed and armored goons down into the Firefly Cellar (whether to cure the afflicted to exterminate them before the magical singing contagion could spread, who can say?) but they found that the problem had already been decisively dealt with by Rufus and Tobias.