So I ran a little expedition into The Stygian Library last night. Just one player, my partner, who seemed to have fun. Here's how it went down.
The system was a weird cludge mostly using bits of Into the Odd, Lamentations of the Flame Princess, and Esoteric Enterprises. Set in the modern day.
The PC was Lucy Farrell, the scion of a wealthy family of occultists, with an interest in demon-summoning. A magic-user with 1 HP, intelligence 17, shit stats otherwise, and the spells Command and Fortune Telling in her spellbook.
So she's going into the library in search of a grimoire teaching her how to summon and control a particular demon.
She finds a doorway to the Stygian Library in her house's study, hidden behind a bookshelf. It's only three feet high, so she opens it up and crawls inside. Within, there's a dark room full of scrolls, with picture-frames on the walls. Lucy realises she can't see shit without bringing her own light source, and so returns with some candles to see by, very sensibly locking the door behind her. When she gets back, she finds the picture frames contain maps of various locations - some mundane, some mythical. Right away, she ignores the ominous ticking coming from below the floorboards and finds a map of the Library itself from among them (giving her some progress towards finding her goal).
Continuing to investigate, she looks for the source of the ticking, lifting up a loose floorboard to find a cobweb-filled lair the size of a coffin beneath, filled with dead mice. Within, an abandoned satchel, containing some cash and a strange book of esoteric exercises that increases the reader's physical health! She reads it (because it has a hunk on the front cover - Lucy is only 18 after all) and gets +1 dexterity! Then, being a reasonable individual, she picks up the satchel, so she can hand it in to the library's lost & found desk.
As she searches further, she hears a skittering noise approaching; the giant ogre-spider whose lair she'd found is returning. Before it can reach her, she's headed deeper into the library to avoid it.
Deeper in, she finds a room lit by a roaring fire, with a big stone sarcophagus. Investigating reveals that the tomb is that of one Lady Muriel Farrell, who died in the early 19th century. She thinks for a bit, and remembers that Muriel was a daughter of the family like her who went missing, everybody assuming she eloped. She says a quick prayer, and continues on her way.
Her way, as it happens, leads her to a chapel in the library. Here, she finds some candles, and on the pulpit a holy tome dedicated to Orcus, Dis Pater. She reads it and learns Cosmic Secrets that cause her to instantly level up! Since she levelled up reading the revelations of the Lord of Dis, the spell she gets is Speak With Dead. She gets a single extra HP, too!
This gives her a plan. She goes back to her ancestor's tomb, and casts Speak With Dead, calling up the ghost to have a chat. The ghost gives her some pointers on the layout of the library, including the rough direction of the demonology section, and roughly where to find the help desk! They chat some more, and the ancestor tells her that she'd died of 'having her brain eaten by a man with an octopus for a head'. After a long and heartwarming chat, the ghost asks to be dismissed since 'it's dangerous to be a ghost in the library'. Lucy obliges.
As she's preparing to leave, she encounters some nuns, also exploring the library! They're polite and helpful, but cross themselves and pray enthusiastically when Lucy asks if they know where to find books on demonology. Quite why she thought they'd know is unclear. Still, they give her pointers towards the library's help desk, which should help her deal with the lost property she's carrying. The nuns leave in one direction - discussing where to find a book called 'The Adventures Sensuale Et Romantique of Sainte Therese' - and Lucy goes back to her original entrance to start searching other 'shallow' locations for the help desk.
After not too long, she finds it, half-way up a flight of stairs, manned by a Yellow Librarian! She hands the satchel of lost property over, and is thanked by the Librarian, who gives her some pointers on where to find the Demonology section. Her progress score is doing alright at this point! Thanking the Librarian, she continues on her way.
At the top of the flight of stairs, she finds a room filled with statues, and containing many scrolls about ancient medicine. She's just starting reading one when she notices a puddle of rippling ink oozing out from under a shelf towards her; an Ink Elemental! She starts to walk briskly away, the Ink Elemental starts to ooze briskly after her. Pausing, she turns, and casts Command on it, telling it to leave her alone! The elemental passes it's save, and responds by engulfing her in a tide of ink. Maximum damage is rolled. Lucy starts to drown, hits 0HP, and takes critical damage. Lack of oxygen to the brain has given her permanent brain damage (disadvantage to any roll involving thinking clearly - luckily she's still int 17 - and a reminder every now and then that she's suffering 'sewious braim dablage') and causing her to cough up chunks of bloody lung as she starts dying.
She struggles free, staggers back down the stairs - pursued by the ink elemental and the inkblots that trail behind it, and collapses before the Help Desk, gasping out 'help... me...' before falling unconcious.
(During her unconciousness, the Librarian on duty does, in fact, help her - stabalizing her bleeding so she's not gonna die, and summoning more Librarians to fight the monster before it can damage any books. Lucy drifts in and out of conciousness, seeing them defeat the elemental, rip its soul out, turn the soul into a Phantom, and put it in a bottle for later.)
She awakens, and discovers that the magic the Librarians used to stop her dying has had the unfortunate side-effect of aging her by a good decand and a bit, and she's now biologically in her early 30s. And most of her body is permanently stained jet-black by the ink. And she still has sewious braim dablage.
She encounters a floating spell escaped from its spell-book, which encourages her to climb the stairs, casts Spider Climb on her a few times, and is eventually shooed away by a Librarian with a broom. At the help desk again, she helps the Librarian with his paperwork, and then decides to try and find a book about how to fix her braim dablage before continuing with her search for demonology. A few pointers from the Librarian on where to go later, and she sets off!
Upstairs, she finds some more rooms filled with statues, before hearing the approach of somebody fleeing from a swarm of bees! Hiding under a carpet, she hears the bees defeated by... some sort of magic... and makes sure its safe before emerging. Before her, a man with an octopus for a head is idly toying with dead giant bees. They talk, she thanks him for defeating the bees, and asks if he's going to eat her brain. He responds that 'no, you've got some nasty brain damage. It would be like eating a sandwich that somebody stomped all over wearing football boots.' He does, however, suggest that the book she's looking for lies in the Brain Room, which also contains many delicious brains. She asks him what he's doing in the library, and he says he's hear to eat all the brains, and that the Librarians think he's a monster because of it, musing that 'there's so much hatred in the world.' He further suggests that they should go looking together, and Lucy (seeing no other option), goes along with it. The neurovore declares that they are now Friends, and that his name is Urglefloggah the Spine Ripper.
Further encounters ensue. They meet some Librarians trying to fix a collapsed shelf. Urglefloggah goes invisible and sneaks off, Lucy tries to help. One of the librarians goes to get supplies, and Urglefloggah ambushes him around the corner and eats his brain. Lucy seems unphased by this.
A little deeper in, they meet some skellingtons, which helpfully point them in the direction of the Brain Room. As they get close, they are cornered against a portcullis they can't open as a monstrous Shade pursues some obsidian monkeys, killing them before their eyes as the pair hide. They see the monkeys become spiritual slaves to the Shade, and become concerned. Eventually working out how to get past the portcullis, they find a magical shroud that protects the wearer from having their soul damaged, which Lucy puts on, being somewhat concerned by what she just witnessed.
Deeper in still, they find a room filled with phantoms in jars! They decide to rest and recover spells here, but as they settle in they are assaulted by a mass of crawling, twitching limbs! Retreating back to the portcullis, they get to the other side just in time, and close it behind them, trapping the crawling limbs on the other side but also meaning they'll need to find a new route to the Brain Room.
So they set off. A few more, less interesting, rooms are found, including ones where Lucy finds a bottle of Conspiritor's Ink (invisible to authority figures!) and Infernal Ink (good for magical contracts!) before, eventually, they find a planetarium.
Urglefloggah sets himself down to read contracts while Lucy starts experimenting with the Astrolobe in the centre of the room. She establishes, after some experimentation, that if you set the astrolabe back, time goes back for the people in the room. Making a little cut on her finger, she sets time back, and finds that the cut has healed.
This gives her an idea. She sets the astrolabe back to before she was attacked by the ink elemental! Her braim dablage is undone, her skin is no longer ink-stained, and she's back to being 18!
Urglefloggah sees that her brain is juicy and whole again, and starts staring at her thoughtfully.
Urglefloggah passes his Save vs Friendship to see if he cares about Lucy enough not to eat her: turns out she's lunch now. Lucy tries to cast Command on him to make him leave her alone, but he passes his save!
In desperation, as he drifts towards her, Lucy sets the astrolabe back a minute. Urgelfloggah is peering thoughtfully at her again. He makes another Save vs Friendship, and once more, decides to eat her brain. She casts Command again (since she'd gone back to before the spell-slot got used!), but he passes his save once more! As he floats towards her, tentacles twitching, she once more desperately resets the clock!
Urglefloggah peers at her thoughtfully, and then declares "Oh, don't worry, I won't eat you. We're friends! Now, shall we go and find this big room full of brains, and then your demon-summoning book?" Lucy, realising that Urglefloggah has decided not to eat her, figures she's stuck with a clingy, overly-friendly neurovore who is - at least - not planning on eating her, and resolves to accompany him until she can find a way to give him the slip.
And we ended the session there.
All in all, we had fun. The player says they want to continue it, as there are all sorts of things they want to explore or try that they've heard about in passing. So that might happen.