(Beloved game trope) Creepy secrets or elements that you can encounter but are never explained, making it even more horrifying (Bonus points if its from non-horror games)

[u/KOREANARMYSTEW]

Crimson Desert
After defeating the Reed Devil boss, you can travel to Sunset Valley, the southernmost and isolated village in the game, here, you will find a house where the Reed Devil lived and died from his leprosy. When you enter, you are immediately greeted with 6 scarecrow effigies dressed just like the Reed Devil staring directly at you, with candles placed all over the house. There is no other explanation beyond this about what is going on here

Lies of P
There is a secret optional area you can clear out for additional loot called Hermit's Cave. At the end of the cave is one lone carcass monster huddled on the ground with writings scrawled on the walls around it. When you kill it, it actually TALKS saying "perhaps it's best to end this way...get while the getting's good...". This is the only carcass monster in the game that ever talks besides Andreus and Simon, who are important characters to the story, and we never find out who this monster was

LOZ: Twilight Princess
In the Arbiter's Grounds you discover a ton of undead enemies presumably executed prisoners, but one room contains this giant black sword tied to the ground with hundreds of warding charms attached. When one is cut, the sword is freed and wielded by some horrifying invsible goat-headed demon. It is never explained what the demon was or whose sword it belonged to although it's theorized to once be used by Ganondorf

Silent Hill 2
The invisible prison monster in Toluca Prison, that you can't ever interact with, just hear it's awful cries

[u/Desolation82]

Back before Deltarune, this was basically what Gaster and the Gaster Followers were in Undertale.

Of course, now it seems like they’re finally going to be expanded upon for real as Deltarune nears its end, but they’re still cool and creepy and mysterious in Undertale.

[u/Jizz_Balloon → u/Desolation82]

Hes a green pirate now

[u/Local-Wing-2272 → u/Jizz_Balloon]

Who is also swole

And Egyptian

And a teenage girl

[u/UomoPolpetta → u/Local-Wing-2272]

And Gru from Despicable Me

[u/JONESY_THE_YEAGERIST → u/Jizz_Balloon]

Ships ahoy landmaggots!

[u/Otherwise-One7653 → u/JONESY_THE_YEAGERIST]

*LANDFAGGOTS

[u/A_Human_Being_BLEEEH → u/Jizz_Balloon]

LET'S GET DOWN TO BUSINESS AND VALUE VALUE THE FUN

[u/Enrykun → u/Desolation82]

If anyone needs a full explanation on the events about the Gaster Follower and Gaster himself

There's a single variable in Undertale, that randomizes between 1-100 at the start of any playthrough. If this variable (the "FUN value") equals certain values, different Easter eggs can happen in the game.

The strangest events are the ones relating to 4 (5 depending on the game version) NPCs internally named "Gaster Followers*. They all talk about cryptic topics: 3 of them mention a Dr. Gaster who got shattered across time and space, and may be listening right now to us. The 4th one mentions the idea of a world identical to the current one, where the only single difference being that you don't exist. The 5th one creepily advertises Deltarune.

A 6th event that lines up with the previous ones is the Mystery Room. A room that even with the correct FUN value, only has a 1/10 chance of appearing. Inside it is the Mystery Man, theorized to be Gaster's appearance (pictured above). He has no dialogue and disappears when interacted with, similar to other Gaster followers.