Hard Mode

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Activating Hard Mode
Hard Mode is not the same as Hardcore Mode. Killing the Wall of Flesh in the Underworld will activate this mode and thereafter permanently set your world to a higher difficulty, so think twice before killing the Wall of Flesh. You can summon Wall of Flesh by throwing a Guide Voodoo Doll in the lava in Underworld.

World Changes
Immediately after Hard Mode is active, a new biome will be created, called The Hallow. It will reach from the surface to the Underworld. The newly formed path will be diagonal, centering below the spawn point, forming a V or an X of Corruption and Hallow.

Also note that destroying Demon Altars will spawn random blocks of Corruption or Hallow somewhere underground. However, they will also spawn random pockets of ore (Cobalt Ore, Mythril Ore, and Adamantite Ore depending on how many Altars you have already destroyed).

Floating Islands can also be hallowed or corrupted.

The Corruption
Once in Hard Mode The Corruption will become much harder to maintain and will spread faster, more 'thoroughly' (more blocks becoming vulnerable to the corruption including stone blocks and sand blocks) and even underground beyond the initial chasms. New enemies will begin to spawn (with much higher HP and damage than previous foes).

Monsters
The Hard Mode exclusive corruption monsters include:
 * Clinger
 * Corruptor
 * Cursed Hammer
 * Dark Mummy (In the Corrupted Desert)
 * Slimer
 * World Feeder
 * Wraith
 * Corrupt Slime

The Hallow
The Hallow acts very similar to the Corruption as it spreads across the world, taking over a handful of other biomes. The difference is it's not in your world to begin with (the Corruption(s) is randomly generated upon world creation). Unlocking Hard Mode will also unlock the Hallow (randomly created, taking over a considerable chunk of a pre-existing slot of land) The Hallow cannot pass through Corruption.

Monsters
The Hard Mode exclusive hallow monsters include:
 * Chaos Elemental
 * Enchanted Sword
 * Gastropod
 * Illuminant Bat
 * Illumunant Slime
 * Light Mummy (In the Hallowed Desert)
 * Pixie
 * Unicorn

Standard Biomes
Once Hard Mode is activated, additional monsters will also appear in the pre-existing biomes.

Monsters

 * Angler Fish
 * Clown
 * Werewolf (Only on Full Moons)
 * Digger
 * Giant Bat
 * Green Jellyfish
 * Heavy Skeleton
 * Mummy
 * Possessed Armor
 * Skeleton Archer
 * Armored Skeleton
 * Toxic Sludge
 * Wandering Eye
 * Mimic (can spawn pre-hardmode but only by activating chest statues with wire)

Bosses & Event
There are three bosses and one event that come with Hard Mode: They are all revised and beefed up versions of old bosses and events. None of these bosses should be taken lightly, and generally would require more than one player to defeat within their night time limits.
 * The Twins (boss)
 * The Destroyer (boss)
 * Skeletron Prime (boss)
 * Frost Legion (event)

Wyvern
Wyvern is a mini-boss. He'll spawn when you are at an adequate height (Floating Islands for instance.) Also only spawns after Hard Mode is activated.

Blood Moon
Blood Moons feature more dangerous enemies in hardmode. The Clown, a hardmode enemy which throws Bombs, can potentially destroy player buildings as well as the environment. This is typically prevented by creating a barrier of indestructible blocks such as Dungeon Bricks.

Additionally, ghost-like monsters called Wraiths will spawn in greater numbers during a Blood Moon. They are a great threat as they pass through walls and are difficult or impractical to kill with traps. However, Wraiths can be dealt with by building a floating house, or otherwise creating a large gap between the player's house and its surroundings. This prevents Wraiths from entering the house by taking advantage of the fact that they cannot float higher than ~3 blocks off the ground in open spaces.

Recomended Preparations

 * It is recomended to have Molten armor and a full molten set before killing the Wall of Flesh.
 * You should always make a Molten pickaxe before entering hardmode so you can gather cobalt as fast as possible.
 * Ranged weaponry is a "must have" before entering hardmode.
 * Have a Nurse, A Merchant, and a goblin Tinkerer Before Entering hardmode.