Break into their house, and wait until they are sleeping. Dress up in a ghoulish Halloween costume that makes you look like a demon. While they sleep inject them with a mild paralytic and a strong hallucinogen, just enough to render them incapable of fighting back and make sure your demonic appearance seems real.
The injections will no doubt have woken them but if not wake them up now. Then go to work... Smash the bones in their fingers and toes, hammer them down to powder, so they can never use them again. Pierce their eardrums so the last thing they ever hear is their own blood-curdling screams. Gouge out their eyes so the last thing they ever see is their own brutal demise. Cut out their tongue so the last words they utter are their own anguished cries.
Then tend to their wounds so they don't bleed out, then call emergency services to have them saved. With no fingers to write or tongue to speak they will never be able to tell of their ordeal, with no ears to hear or eyes to see they will not be able to be given any comfort, deprived of their senses there will be no new sensations to dull the memory of their torture.
Having endured that torture at the hands of a demon whilst hallucinating they will almost certainly have been driven insane, you will leave them a prisoner of their own decrepit body forever tortured by their own tormented mind.