The Dungeon Guardian can fly right outta nowhere, One-Hit KO you, is incredibly fast, and is a gigantic skull. Add that to their ridiculous HP and you have one heckuva nightmare.
The Corruption. Full of Eldritch Abomination enemies and disgusting-looking terrain. And it spreads. Moreso if you've killed the Wall of Flesh and triggered hardmode. The eerie music does not help the mood either.
The Crimson from 1.2 is far worse. Filled with horrific enemies which embody the concepts of Body Horror and Nausea Fuel with a passion and its armor/weapon set is incredibly gruesome. And the grass is blood-red, with meaty "stone" and darkened wood. And the music? Scariest track in the game by far.
It's lessened/stengthened if you realize that the entire biome has trappings of a "haunted game" creepypasta. A strange area of the game that shows up at complete random, and is jarringly out-of-place considering everything around it.
The background itself is extremely unsettling. While the Corruption at least has foliage in the background, the trees in the background of the Crimson look lifeless and have bits of bloodied flesh hanging off of them. It's like the land itself is sick.
The soundtracks for the Corruption and Crimson can be quite unexpectedly creepy for the unprepared. In particular, the more ominous Underground variants of both these themes can sound very oppressive and foreboding.
The bosses can be quite scary to those who do not expect them.
You've found the Dungeon's entrance, and then you talk to the old man who warns you about his curse, when all of a sudden he explodes into a gigantic skeletal menace.
One evening, while setting out to do some night-time Star gathering, you receive a cryptic warning about being watched, and then you hear a deep, menacing roar and this giant eyeball flies out of nowhere.
1.2 introduces Brain of Cthulhu to the Crimson biome. Going to the Deep Crimson to smash beating, exposed Hearts for treasure? Be careful what you awaken... a monstrous brain that summons eyeballs and teleports while attacking you in various ways and roaring!
The Wall of Flesh can be quite the shock for any player unfortunate enough to fail in catching the voodoo doll, if they unwittingly and/or accidentally kill a Voodoo Demon over the lava... The Wall of Flesh inflicts the Horrified debuff on players when it spawns, so its fear-inducing status is coded directly into the game.
Just beat the mechanical bosses? Going to look for the new stuff in the Jungle? Don't break the glowing purple bud - unless you want to fight Plantera, the giant-sized aggressive plant boss. Avoid luring it out of the Underground Jungle, too, because it will then go berserk and start attacking more aggressively.
Hell is somewhat creepy, as to be expected. But the scariest part is the music, fittingly titled "Eerie" on the OST.
The Eater of Worlds. Full stop. The thing looks scary, and if you cut it in half, YOU END UP FIGHTING TWO.
How about a new player who doesn't know what smashing Shadow Orbs does? "Hmmm, another Shadow Orb. What's this one drop? Wait - The Eater of Worlds has Awoken? What's that me-" GIANT WORM OUT OF NOWHERE.
The console version's final boss, Ocram, is hands down the boss most worthy of being called an Eldritch Abomination. His as-yet-unreleased music doesn't help either.
In Hardmode, smashing a demon altar can cause a single block anywhere in the world to become corrupted/crimsoned or hallowed. There is NO indication of this when you smash the altar, so anyone who read this and didn't know beforehand just had a My God, What Have I Done? moment.
As of 1.2, the three mechanical Hardmode bosses have a small chance of naturally spawning every night.
"You feel the air growing colder around you..."
"You feel vibrations from deep below..."
"This is going to be a terrible night..."
The nightmare fuel status of these three grows the more you play Terraria-you know these bastards are coming. You know they want you. But you won't know when, where, or if you'll be geared enough to kill it.
The Solar Eclipse from 1.2. Extremely fast, tough, and hard-hitting renditions of classic cultural monsters quickly hunt you down on the surface. Without the new high-end equipment introduced in 1.2, you are likely to run into difficulty surviving. Oh, and since a Solar Eclipse takes place during a day portion, it can happen right after or before a blood moon.
In fact, a Solar Eclipse has a higher chance of occuring after a Blood Moon. And vice versa.
The Crimson, which is a variant that will take place of the Corruption upon creating a new world. Dark, eerie background music and has a monster that gives out a very different snarl which is likely to cause a Jump Scare to a new player especially if they are close.
Spider Nests are this for those with arachnophobia. The worst part? A NPC is held captive there.