Hardmode

Unlocked after killing the Wall of Flesh, Hardmode drastically increases the level strength of all biomes, introduces new monster spawns, and new biomes including the Hallow and Underground Hallow.

Demon Altars become destroyable and, when destroyed, individually spawn the ores Cobalt, Mythril, and Adamantite into the world.

Items
Health Crystals may be used after level 50 to increase max health past 400 up to 800.

Magic Crystals may be used after level 50 to increase max mana past 200 up to 300.

These two items are only valid on pre N Terraria 4 versions.

Bosses
When attempting to spawn regular mode bosses in a hardmode world, you will instead find these come to muck up your night.

Eater of Worlds = Devourer of Planet Skeletron = Dungeon Keeper Eye of Cthulhu = Vengefull Eye of Cthulhu
 * It became much bigger
 * It grabs you with its hand and throw you somewhere.
 * It grabs and leech the monsters health
 * Multiple Stage Boss
 * Releases boulder on second form and chasing.
 * Launchs cursed fire in 8 directions when rushing on first form.

These are also some of the classic hardmode bosses you will encounter:

The Destroyer- Much longer, mechanical version of the Eater of worlds. Does not split, spawns Drones when hit and shoots lasers from it's body.

The Twins- Two more powerful, mechanical Eye of Cthulhus. They are attached. The red eyed Retanizer will shoots lasers at you, and occasionally rapid fire them. The green eyed Spazmatism                     will spit balls of Cursed Flame at you and try to stay horizontal with you. When either of them get below half health, they will become much more powerful. Retanizer will shoot rapid                       barrages of higher damaging lasers at you, while Spasmatism will charge faster and further with more damage, and also spew Cursed Flame, much like a flamethrower would, but                          gives the Cursed Inferno debuff instead.

Skeletron Prime- A much stronger, four armed, mechanical version of Skeletron. Each of his hands have different methods of attacking. One hand is the Prime Cannon, which will fire flashing                                   bombs in the air towards the player's general direction. The Prime Laser shoots lasers at the player, similar to Retanizer. The Prime Vice and Saw will swipe over the player,                                   dealing hefty amounts of damage on contact. Skeletron prime will occasionally enter "Rage Mode", when his head starts to spin, gains spikes but moves much slower. In this                                   form, his limbs will also act differently, with the Cannon firing faster directly towards the player, the Laser firing constantly, the vice frequently lunging toward the player and the                                Saw which directly homes to the player's location.

Plantera- A large plant-like creature which will climb around in the underground jungle, using it's legs which are much like hooks. In it's first form, it will shoot pink seeds at the player, while when                 taking more damage they will become poison seeds, and eventually add thorn balls which bounce around. In the second form (when reaching half health) it will attempt to make constant               contact with the player, but will also spout smaller mouths called 'Plantera's Tentacles'. It will also shoot small spores at the player, similar to the spores shot by the Giant Fungi Bulb. Defeating Plantera will result in the Skeletron Dungeon becoming much more difficult, with extremely harder variants of skeleton inside.

Golem- A large, (you guessed it) stone golem. Upon defeating plantera, you will receive a Temple Key, which can be used to enter the Lihzhard temple and defeat Golem. In it's first form, golem               will shoot ricocheting fireballs, extend his fists toward you and jump around. When the fists take more damage, they will punch faster, the head taking more damage will make golem jump             around more. In the second form, the head must of taken full damage, and then it's head will detach and fly around at high speed, raining down lasers from above, while also becoming                 invulnerable. The head will continue to shoot fireballs, and if the fists have not been destroyed, they will continue to attack with increased speed. Only the body of Golem needs to be                     defeated in order to receive the drops.

Duke Fishron- Currently the final boss. It is a fishlike pig creature which flies around at great speed. In it's first form, it will lung at the player five times, before doing one of two projectile attacks. One projectile can be the detonating bubble, which is when he will spew 10-20 bubbles from his mouth which will follow the player and deal damage on contact, however they can                           be destroyed with any attack or projectile. Another projectile attack he can do is the Sharknado, which is when he shakes his fins, which summon liquid blobs which will fall, creating                       a large cyclone. This cyclone will shoot Sharkrons at the player, but these sharkrons can be killed. He will lung five times again, and repeat the cycle until put into the second stage. When he reaches half health, he will go into the second stage, he will stop for a moment, then become darker and his eyes will glow. In this form, he is much faster. Before                                     attacking, he will only lung three times and begin one of the two upgraded projectile attacks. His detonating bubbles are fired when fishron spins in a loop. The Sharknados become                       Cthulhunados, the blob will follow the player, attempting to spawn the Cthulhunado on the player. Cthulhunados are much larger than the Sharknado, and will create twice as mant                         Sharkrons as in the first stage.