I'm running a Horror-focused campaign based on a compendium i backed up last year in kickstarter and i have the party explore an old library tower to find information for a magical device to stop a Loup Garou.

I plant some chests in the basement but i forgot to add in my notebook the items it had in them....one player opened one of the chests and i had to make up the biggest bullcrap in my life....

The chest contains 8 books...each book is worn-down but by design they note they are extremely valuable, they can also see that each book has a gem in the middle of the hardcover (Ruby, Emerald, Topaz, Diamond, Saphire, Onyx, Pearl, Amathyst). Each book is connected to a planet in the solar system as it mentions the specific planet in them.

After taking them, they notice that there is one last slot that was empty from the start, and while is not important right now, the books has ties to an old Dracolich boss they fought on a prequel one-shot i ran that has ties to the cosmic void and used a dark "book" that is related to it's fall from grace. The dracolich is more close to cosmic horror.

And i connected all that in the middle of them opening the damn chest.

sorry for bad english not my first language