For those of you who don't know who Cioccolata is, he's basically a crazy doctor who loves to record snuff films and make people suffer for his amusement. His Stand, Green Day, basically releases a kind of mold that, if you go down to a lower altitude, will start growing on your body and will literally destroy your body completely if you go down too low. I'd post a video link to his backstory here, but I don't think fimfiction would like it if I posted the video of Cioccolata's backstory.
Here's a picture of Cioccolata.

Hmmm, it was stated that at one point that Cioccolata worked at a nursing home and drove the elderly to suicide for fun. To that end you could have sort of a murder mystery where the immortal Twilight is visiting the elderly mane 6 in said nursing home notices something up as they look depressed each time she visits them and after one of them dies she starts digging and discovers him and his stand.

That or you could have him be an old classmate of Twilight's (as doctors going to a school for magic isn't too out there since unicorn doctors would need good control over it) and after noticing he's off looks into it more.

Finally in Equestria Girls a friend of Sunset's (mane 6 or otherwise) comes down with a "rare fungus infection" after being treated at hospital and Sunset, knowing it's supernatural in some way, investigates

This would be an interesting displaced fic.