Hard Mode

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Activating Hard Mode
Hard Mode is not the same as Hardcore Mode (where death will delete the player automatically). Killing the Wall of Flesh in the Underworld will activate this mode and thereafter, permanently set your world to a higher difficulty.

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 spawnpoint, 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, Mythril, and Adamantite depending on how many Altars you have already destroyed).

Floating Islands will also be hallowed or corrupted.

The Corruption
Once in Hard Mode The Corruption will become fiercer to maintain. New enemies will begin to spawn (with much higher HP than previous foes) it 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.

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

The Hallow
The Hallow acts very similar to the Corruption in that 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).

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
 * Werewolf (Only on full moons)

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

Monsters

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

Bosses & Event
There are 3 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 1 player to defeat within their night time limits.
 * The Twins
 * The Destroyer
 * Skeletron Prime
 * Frost Legion (not able to be fought after the new year)

Wyvern
Wyvern is a mini-boss. He'll spawn when you are at an adequate height (Floating Islands for instance).