As I worked on the hexcrawl, I had thought about a location that includes someone caught in an imprisonment spell, bound in chains. So here’s a small dungeon where the chained one resides.
Whoever imprisoned the chained one put doors on the entrance, with wards and locks. Since then, something has made a lair in the entryway, and guards both the door and the exit of the underground stream. The doors can be bypassed if you travel upstream, because an earthquake sometime after the imprisonment opened a gap in the wall between the outer prison chambers and the river.
I imagine a cave fisher or a roper in the cave with the rubble, ready to catch a fish or bat or adventurer. There might be an additional guard set by the caster of the imprisonment, perhaps a golem or some other construct. Depending on the background of the prisoner and their captor, the guard could also be a celestial or a fiend.
The actual prison chamber has a raised section on which the prisoner lies, bound in many heavy chains. A wall of stone blocks the river corridor to the west. Writing on the wall outlines the many crimes the chained one committed, and possibly contains a clue to ending the spell. Getting to the back of the stone wall requires navigating the rocky rapids just downstream.
Now, I have to determine who the chained one is and why they are imprisoned here. Is the person who cast the spell still alive after centuries? What would be the reaction of the chained one if the adventurers free them? Perhaps if I get to GM the hexcrawl, I’ll figure that out.