The first three bosses after the Wall of Flesh are robotic versions of the previous bosses.You're rebuilding them to become more powerful and more destructive. As of 1.2, there is a random chance of these bosses spawning on their own if you have not killed them yet. Perhaps someone else on the island decided to reconstruct them first. Official lore reveals that the Mechanic was forced to build them while trapped in the Dungeon. But this only raises questions as to what's in the Dungeon that's capable of being used to build stronger versions of one of Cthulhu's eyes, a gigantic worm that lives in the Corruption, and a skeleton that can become near-invincible in the day.
We have: traps that are placed pretty randomly into underground locations, mineshafts and dwellings that pop up in some pretty deep locations, a large, fleshy wall with two eyes and one mouth you summon by throwing a voodoo doll of the first human you meet, and somebody apparently rebuilding large monsters using machines. That means nothing good could have happened to those who took up underground.
There are werewolves at night-time. What are werewolves? People. You're on an island with few people to help you, and you're really killing them.
The Clentaminator can be this. Think about it, you're a creature living in the Crimson/Corruption biome, then some random adventurer starts destroying your home, turning the grass blue, the stone white, and then you see all the people you used to know being slaughtered in front of you...
If one looks closely, the Crimson background features what appears to be mountain sized bones, such as a skull and a ribcage.◊ That and the addition of ichor, seems to be evidence as to how the Crimson came to be.
If the Crimson's origins is indeed what the statement is above, then it growing larger might be a VERYBADSIGN. The official lore since stated that the Crimson is its own Genius Loci that spans across all worlds and is not the regrowing remains of Cthulhu, which in many ways in actually worse. Consider how many copies of the game were sold, how many worlds each player made on average, and then take in to account that half of them at least have a little bit of Crimson. Should it somehow "unite" all of that controlled land, and the Crimson would be astronomically powerful.
The reason why the Cultists are able to summon the pillars and the Moon Lord? You. They only appear after you unlock the Lihzahrd Temple and destroy its protector, the Golem. With the temple open and lacking a protector, the cultists are free to waltz in and steal the tablet, hence triggering the Lunar Events. Nice Job Breaking It, Hero!.
You can stop pre-Hardmode Corruption from spreading by planting sunflowers. In real life, folks living in Japan plant sunflowers to absorb radiation from the nuclear plant that was flooded in the tsunami.
They did it prior to that in Ukraine to help purify the soil contaminated by the Chernobyl disaster.
Sunflowers have long been planted around the edges of fields, especially among the Pennsylvania Dutch community, to ward against evil.
The enemy types at each Celestial Tower correspond to the class equipment its fragments give you:
Solar Pillar (Melee): Enemies mainly attack by Collision Damage or rolling into you.
Vortex Pillar (Ranged): Enemies attack with ranged attacks from a distance.
Nebula Pillar (Magic): Enemies attack with various energy bolts and debuffs.
Stardust Pillar (Summoning): Enemies are able to spawn smaller enemies or Attack Drone-like objects.
In 1.3, the Lost Girl/Nymph now drops a Metal Detector. Considering that her behaviour is based off The Fair Folk rather than an actual Nymph, and Fae are harmed by Cold Iron, she's probably using it to avoid such substances.
The Merchant can sell Dirt Blocks for high prices on the international market because there are countries that buy them to create arable land for farming.
Why is the Solar Pillar the one that represents Melee? Because out of the things the Pillars are named for, the sun is the nearest to us.
Why does Skeletron Prime become so inhumanely hard at dawn? That's probably how he's supposed to be, but he's solar powered. So when you fight him at night, he's running on auxiliary power and has to tone down his weapons to not drain his battery too fast.
During the course of the game, the player fights three giant eyes in the forms of The Eye of Cthulhu and The Twins. Three eyes seems like a weird number, especially since The Twins are the only mechanical boss to incorporate two versions of its Pre-Hardmode variation. When fighting the Moon Lord, the player releases three True Eyes of Cthulhu in order to expose his heart.
All of the chlorophyte-made armors are all related to some form of life: Chlorophyte armor itself is based on plants, Turtle and Beetle armors are based on animals, Shroomite armor is based on fungus, and the Specter armor is based on death. The reason for the theme? Chlorophyte is said to be a "living ore" in the PC achievements.
Summoning the Wall of Flesh, which enables Hardmode when defeated, involves killing the Guide. The Guide's main purpose is to help new players learn the ropes; by fighting and defeating the Wall of Flesh, you're proving that you're capable enough to handle what happens next and thus don't need the Guide to hold your hand anymore.
King Slime isn't the slime itself; the ninja within is controlling it. Not only does the achievement title, Slippery Shinobi, imply the ninja is the one in control, his Trophy is the ninja's sword covered in slime, and the Expert Mode drop for King Slime explains just how the ninja was able to accomplish this—Royal Jelly makes all slimes throughout the game become friendly and deal no damage once equipped.
The Grand Design is crafted from items bought from both the Mechanic and the Goblin Tinkerer, which definitely shows their budding relationship with each other.