>>33458882>A variety of Ghost Pokemon, obviously>Quiet music with "bumps" but no obvious rhythm>A single NPC that can't be reached or interacted with (maybe the Player's sprite?) that's only present your first time through>No other NPCs at all>Pathways that look similar>A long, featureless, "flashable" hallway at the endThe NPC could have dialog like "......" to prevent crashing if the player cheats their way to them. Avoid calling the Player out as a cheater, since it'd break immersion. But also don't reward them with additional dialog.
The final hallway would be a dark room with flat walls, requiring Flash to see.
It should be long enough to make the Player question it, but not long enough to be annoying (maybe a good 30-45 seconds of walking?).
You could have the room be entered through a hole in the previous floor so the Player can't go back, adding to the "spooky" atmosphere.
Somewhere along the walls in the final hallway could be a strength rock or a small lake that the Player would only see if they came back with Flash.
Past that could be a Ghost-related item like a Spell Tag, or even a battle with the aforementioned NPC, who would really be a high-leveled Ghost Pokemon with a special ability or moveset that would make them worth catching (Gengar with Prankster or Shadow Tag, and moves like Mimic or Role Play).