To quote wikipedia:
Prominent features of Gothic fiction include terror (both psychological and physical), mystery, the supernatural, ghosts, haunted houses and Gothic architecture, castles, darkness, death, decay, doubles, madness, secrets, and hereditary curses.
The stock characters of Gothic fiction include tyrants, villains, bandits, maniacs, Byronic heroes, persecuted maidens, femmes fatales, madwomen, magicians, vampires, werewolves, monsters, demons, revenants, ghosts, perambulating skeletons, the Wandering Jew, and the Devil himself.
There are cursed travelers who wander the countryside, trapped in the early morning twilight- never again will they see the sun rise. If you encounter such a traveler while making your own way in the early morning, their loneliness will drive them to accompany you- perhaps asking you to guide them or saying that they're headed to the same destination.
When the sun rises, the traveler will disappear- and so will you, if you walk or ride alongside them. Of course, you won't perceive this- instead, you'll wonder where your companion could have gotten to, and perhaps be surprised that the sun hasn't yet risen. . .
When the last female of a species in a given forest is killed and eaten by a predator, that predator is cursed. It does not sleep or age, and is compelled to stalk, kill and consume its own kind. Over time they inevitably become very, very good at this.
Animals can instinctively sense when a prey is sacred, and thus rarely bring such a curse upon themselves unless starving or otherwise deranged.
Humans are a different story.
They call it the confessor- a figure in inquisitor's robes, face concealed by a large hood. It can cloud a man's mind, induce him to admit his deepest, most shameful secrets and desires. It seeks those whose inner natures are the most foul and vile- and consumes them in a very bloody fashion. Only then does it throw back its hood, revealing the face of the man it has just slain- for its true mission, as a servant of the devil, is serve as a vessel for the evil within men and women so that it might be unleashed on unsuspecting friends and family.
Sometimes a body becomes a part of a household, literally. A corpse is concealed under the foundations as they're being laid, or behind freshly-laid section of brick, or a child goes exploring in the spaces between the walls only to be lost and never found. You'd be surprised how many castles have more than one.
These bodies do not rest easy. They can move throughout the walls and floor of a household, even when there shouldn't be nearly enough space to accommodate them. Keep a sharp ear out for the sounds of scratching movements, be they in the walls, floor or ceiling. And if you see a new opening in your house, stay well away. Doesn't matter if it's a missing brick in a wall, a broken section of floorboard, or an opening in the drywall as though some drunkard threw a punch- come too close and a cold dead hand will be waiting to seize you and drag you inside.
Of course, I should mention that they like to make these holes in places where they can't be seen- behind a mirror or painting that's hanging on the wall, or under the edges of carpets and beds. . .
If a woman is scorned so badly that she takes her own life, the result is a particularly malicious type of spirit- one that does not possess humans directly, but instead works through inanimate objects. A man in the presence of a fair maiden who takes hold of a knife or fireplace poker might abruptly change his demeanor and attack her in what can only be described as a jealous rage. These spirits focus on one such woman at a time, stalking them until the victim is finally dead. . .
There's a rare breed of revenant that arises under certain conditions. It's generally agreed that one of these conditions is that in life they committed murder, but no one's quite sure what other criteria must be met for them to rise again in this manner.
This type of revenant appears to be perfectly healthy and has a pulse, though those who play close attention may note that they do not breathe. The creature retains his intellect and memories, except that any motivations he may have held in life are replaced by the urge to do murder for murder's sake.
The creature has the ability to detach any part of their body at will, including their head; the section in question literally rots away, reverting to the dry and dessicated form it held in the grave. Despite its withered state, these sections are fully capable of movement and act autonomously as their owner wills.
The only part that cannot assume this detached state is the creature's torso- more specifically, its heart. This is the creature's weak point, and impaling it is the only way to stop this breed of revenant for good.
There's a certain set of words that, if said as one continuous sentence, will cause you to be struck dead on the spot. Your tormented spirit will wander the earth forevermore, speaking that same fateful phrase over and over. At first your words will be perceived as a distant muttering, but every time someone tries to listen the words will be louder and clearer to them. And once they've heard and comprehended the entirety of the cursed phrase, the urge to repeat it themselves will begin to grow. . .
So if you're alone one night and hear a distant voice, resist your curiosity at all costs. Each time that you fail to ignore the voice will only make the next time worse. . .
The devil has a favorite deal he makes with those willing to sell their soul for a shot at immortality. He grants them the power to strip the skin from a victim's face and wear it as though it were their own face. And he informs that that from now on they will age a year each day- but so long as they wear such a mask, it will age in their place.
Hardly ideal, but it's still a decent bargain- after all the devil's ownership of your soul only matters if you die.
There are things out there, formless shadows. Maybe they're evil spirits, maybe they're something else entirely. All I know is that they rely on mankind to give them form. When a storyteller (or perhaps a would-be prankster) dreams up some manner of nightmarish bogeyman, it listens to their thoughts and slowly takes the form of that very idea. By the next full moon, they've almost become real, and anyone can perceive them- but their form still has no substance, and will pass right through any would-be victim. . .except that of their creator. Once they slay him, they're free to murder and wreak bloody havoc in whatever manner their creator imagined them doing.
Or at least, that's what the case will be if they can manage to kill me.