I'm writing a scene for my blog where the PCs are prisoners of a black dragon, and being the sadistic monster that he is, he will attempt to kill the PCs in different "Oh, that was an accident" type ways. He's playing with them instead of killing them outright. (you must have read a novel where something similar was done, surely!)

I have three ideas, but am looking for one more to finish fleshing it out. Can anybody suggest something?

My ideas, for inspiration: 1) He locks them into a large warehouse, or other large wooden building. On the way out, he "Accidently" breaks the oil lamp, setting the building on fire. 2) He imprisons them in a hole that just happens to contain one or more oozes (fighting them in a hole should nicely increase difficulty). 3) He lets them go. During a pouring rainstorm. Onto an island in the swamp that is about to get flooded. If they make it to safety, he changes his mind and recaptures them.

Thank you very much

thinking they've fooled the evil one with costumes, the party sneaks down to save the princess. they 'escape' through a narrow shaft into a pit. add a some trash, some wet slurry, a tentacle monster, an old couch, and then start the walls closing in. party will need to remember their sending stone was shared with the clumsy translator elf and his violent, mumbley dwarf friend in the control room.

Literal death by a thousand cuts is to my mind a terrifying form of torture/execution. Having delved a little into accounts of Lingchi, it was practiced in many different forms. I don't think I can be sensitive or non-judgemental, but even if this punishment was carried out after death it was a horrifying and brutal practice. It occurs to me that such practice carried out prior to death would be the work of a truly sadistic mind.

For those, who culturally belong to the areas where this form of history is practiced, please know that I'm not trying to pass judgement on said culture - English history is littered with similar cruelties and I do offer apologies if I have offended someone.

He forces them to read Reddit threads, making them believe that every post was done under the effects of a Zone of Truth. 

he's an aquatic thing, maybe waterboarding?

"accidentally" floods their sleeping chamber?

diseased drinking water?

what disease?

sight rot?

slimy doom?

something else?

There was an idea i saw in a youtube video of having a dragon use magic to impersonate a dragon slayer and giving false information on how to slay a dragon. For example by saying there is a dragon slaying item  at the end of a trap filled path the dungeon that is either something the dragon wants or will make them stronger. The specific example was a root in a dangerous forest that they claim weakens the dragons acid breath but would make it stronger.  For a prison setting it could impersonate another prisoner and give them a bad escape plan. 

Vengeful ghosts in their cell.

Think 'Cat and Mouse'.  He has them trapped, leaves room for an escape.  Allows them to escape, just to recapture at the end. 

Or maybe a trapped looped inside their heads like a dream.  They go down the tunnel, go right just to fall into a deep chasm, wake up back in the cell they were in.  Next time the go left, fight a army of undead, just to wake up back in the jail cell.  Next time they take the middle path.  a few traps that they get by, an easy monster they kill.  They see a light at the end of the tunnel, their freedom.  They get to the light, it's a 1000 feet high with shear cliff side all around them.  They see a small black dot off in the sky line,  the dot is getting bigger bigger, it is the dragon flying right to them.  They are in a long narrow tunnel with a breath weapon that can blast them. Is it an illusion like the last 2 paths?  Is this one real?