My party are in a dungeon right now which contains an encounter I am really looking forward to.
They are currently in the sewers of a city, trying to rescue a spy who was infiltrating a cult they have had previous dealings with.
The sewers are very inspired by the Paris Catacombs, a hodge-podge of different tunnel systems and rooms created at different time periods for different reasons, so there is stuff there entirely unrelated to the cultist squatters.
This is where the encounter I am anticipating comes in. In one of the rooms of the labyrinthine sewers, behind a door far older than the slapdash attempts at construction the cult have been engaging in (just taking furnishings and doors from their houses in the city and setting them up in the sewers), there is a single dog skeleton.
Upon entering the room, an apparently hostile ghost will appear - a large, spectral hound; snarling and defensive.
The party could of course just fight the monster, it wont be the most deadly thing they will find in the sewers by a long shot, but they dont have to.
They have a gnome wizard, who can speak with animals, and who is desperately Lawful Good and determined to do the right thing. So it is entirely possible (and what I hope happens) that she can convince the ghost that they aren't a threat and find out what its deal is.
Its deal is, by the way, that it was lead down here and abandoned a couple of decades ago by a family who were being evicted and couldn't afford to keep it.
Befriending the ghost dog instead of fighting it will cause it to act as a guide of sorts through the dungeon, acting as a dim light source and possible lookout if they rest.

If the ghost dog survives the dungeon, and they take the collar from the skeleton, then it will become a magical item:

Once per long rest, they can activate the collar and summon the spirit of the dog for an hour.