Throw it into OpenAI and let it come up with something 
I used the following prompt:
An idea for a dungeons and dragons campaign titled “Bad Shadow”:
And it spat out:
The PCs find themselves in a dark and dreary dungeon, beset on all sides by strange and dangerous creatures. They must find their way through the maze-like dungeon, avoiding traps and solving puzzles, while fighting off the never-ending stream of creatures that seem to be drawn to the darkness.
As they progress, they begin to realize that there is something more to this dungeon than meets the eye. They find themselves being followed by a strange, dark shadow that seems to be feeding off of their fear. This shadow seems to be the source of the creatures in the dungeon, and the PCs must find a way to destroy it before it consumes them completely.
The twist is that the shadow is actually a manifestation of the PCs’ own fears and doubts, and it is only by facing their fears head-on that they can hope to destroy it. They must overcome their own personal demons in order to triumph in this dark and dangerous place.