That's actually a slightly altered (to remove the title) pic of a cover scan of Papus's fin de siecle occult work The Tarot of the Bohemians. I found it by doing a google image search for "grimoire cover". When I started work on this adventure I just wanted a cool graphic of a spellbook to show the players, but that image spoke to me such that I ended up writing the whole scenario around it. The dungeon was shaped like a spoked wheel and most of the encounters were with the creatures depicted. Though they never fought the winged cow. Every time they heard the menace of its mooing the party fled in the opposite direction.
Anyway, Alvis the Pardoner assumed the azure tome was Tigerblood's spellbook and made a grab for it. That set off a curse-type trap that sucked the party (including the Welsh brothers, who I ruled had been with the group the whole time when Karl showed up slightly later) into the Accursed Demiplane of Atarota. The goal of the session now became simple: get back home.
The aesthetic of the Accursed Demiplane was inspired largely by dream sequences and other planes in various old cartoons. Two examples that come immediately to mind are the dreamscapes in the Hugo Strange episode of the Batman: The Animated Series and any of several brief planar excursions in the D&D cartoon. I think the best example from the latter is from the episode with the gnome illusionist and her magic treasure chest. Anyway, imagine a big cosmic-fueled outer space, not the real empty kind, but the Kirby crackle-riddled and meteor-strewn comic book version. Add a flat plane of transparent material for the party to stand on. Marvin the Martian's space platform/observatory/artillery battery is another example I just thought of.
Build a ceiling-free dungeon on top of the glass with corridors 100' wide and hundreds of feet long, chambers more vast than the eye can readily take in. Use for construction material vast, cyclopean slabs of an unknown matte black substance, reminiscent of the House of Leaves. And then populate the dungeon with stuff, like the Ferris Wheel of Blood, the Clockwork Torturtron of Nubazel the Damned, the Shattering Jewells of Avarice, the Hole to Hell, the Orchard of the Serpent and the Phoenix, the Cloudery of the Weeping Angel, etc., etc. You know, the kind of stuff that should go in every dungeon but that just seems all the more odd because the whole edifice is suspended in electric outer space under the vast Sleeping Eye of the Watcher, which looks kinda like if a spiral galaxy was an open eye but this one is shut.
The players quickly intuit that the Fountain of Satyrs is some sort of device that can maybe get them home, if they can find the four strange runes that fit into four slots on the fountain. They confirm this guess by consulting a strange metal book in the Library of the Riddleless Sphinx, who Karl later stumps with an unsolvable riddle, winning both directions to the fourth rune and the Sword of the Sphinx in the process.
So the players start crawling this out-of-scale and out-of-mind dungeon to claim all four of the Runes of the Waiting Riders, who can take them home if summoned. Along the way they release a Blood Ooze from his prison in the Ferris Wheel, but quickly dispatch it with magic. Alvis the Pardoner nearly has his face blown off when he tries to cop one of the Shattering Jewells. They flee from the mooing and clanking cowbell because they first hear it in a standard Room Fulla Statues That Are Obviously Petrified Dudes. Good call. That cow was really a rare Gorgon Heifer. At one point the party discussed navigating the room while blindfolded in order to avoid a gaze attack. They must have seen the look of glee in my eye at the prospect.
The party opted to free Nubazel the Damned from the Clockwork Torturtron, a device designed to grind him between its gears for all eternity. This Anubis-looking weirdo tried to sneak into Heaven and the Torturtron was his decreed punishment. On the plus side Nubazel gives the party one of the four plot tokens they need, on the minus side they've directly confounded the dictate of Heaven. In case no one Upstairs was watching that, Alvis steals the golden apples from the tree in the center of the Garden of the Serpent and the Phoenix. He also breaks open the egg he finds in a nest there, killing the embryonic phoenix inside. And then he goes on to try to steal an angel's harp. All with only 2 hit points to his name.
Another encounter I liked was with Spyros the Giant, a dude I adapted from an old Son of Satan comic. He looks like a giant in cheesy bondage gear like the villain in the second Mad Max movie but with an executioner's hood and a big mofo battleaxe. His deal is that all the sins of humanity course through his veins like black, viscous poison. I decided that he'd help the party if they'd confess all their sins so that being in their presence doesn't wrack his body with nonstop pain. Honestly, most of the players didn't seem to know where to even start cataloging their numerous transgressions, so they end up having to kill the poor son of a bitch to get past him. When he falls over dead the party's worst suspicious are confirmed as the floor starts cracking under him. They run like hell and look back just to see him fall through the shattered glass down into an infinite void.
Eventually the PCs put together a complete set of Runes of the Waiting Riders and insert them into the Fountain of Satyrs. The satyrs stop blowing water out of their pan-pipes and instead play a song that summons the Riders, fiery interstellar centaur ladies who wordlessly transport the party to just outside the castle where they started. Which by now is mostly on fire. I agree that they can save the spellbook whose curse they spent the night evading and we call it a session.
All in all I thought it was a good run. The players hit that magical mix of taking the ridiculous scenario in deadly ernest plus remaining lighthearted enough to walk straight into trouble from time to time.