Nightmare Fule

Lots of stuff, I imagine. It's played off as cartoonish, but the Sims universe is packed with awful stuff. Most of the deaths a sim can experience are pretty violent or gruesome, and even if they manage to avoid all of that stuff and peacefully pass away of old age a faceless dude with a scythe shows up at the house and traumatizes the survivors by sucking the soul out of grandma right in front of them.

What's more, they see it happen over and over again, regardless of how they die. So every sim is painfully aware that this is what is waiting for them and their kin at the end. There's no reassurance of "no Johnny, it's going to be okay, that won't happen to you". Johnny, take a good look because this is exactly what you're in for.

@Bloosmoo Thanks!

Waking up dressed like a random townie. I can't think of anything scarier than that. :D

I agree that the Selvadoradan spiders are nightmarish. Even the little spiders that crawl out into the webs from the Vampires pack are icky. (Arachnophobia, lol. Nice job, devs!)

I also think that the drowning animation is gruesome (thalassophobic as well) because it's really realistic looking and less comedic. But I like that about it.

Even the Strangerville possessed Sims have an eeriness to them that I find exciting. The launch for that pack was epic because players had no idea what to make of the impossible postures and macabre grins. <3

I kind of like to be scared and creeped out by the game. This is one of the reasons I really, really want Sims to feel fear and have phobias. It's so human and relatable.

When you try to make grilled cheese but get burned and died

The whole Strangerville incident. Can you imagine waking up and seeing someone you care about
shambling like a mindless zombie with this creepy smile and bulging eyes?
Now there is a real monster for you.

Spiders and clowns.

Enjoying a nice time in a gym, you know you don't have to sweat yourself, while the death music starts to play and you find out it is one of your sims (not active) freezing to death in Oasis Springs in summer.

There have been many scary things in The Sims series. Red eyes staring from bushes in some The Sims 4 locations, krakens in The Sims 3, meteors in The Sims 3 Ambitions are truly jumpscarish, creepy noises heard from gravestones in The Sims 2, scary soundtracks in The Sims 1 for burglars, getting fired, clown catchers, racoon, death, fire, and ghosts, and the list continues.

I must admit I still get the creeps from seeing those snakes in Selvadorada. I don't mind them about, but I still get paralyzed when they appear, even on those spots where I'm actually expecting them.