Hardmode, as its name suggests, is essentially a more difficult version of the initial world. A world permanently converts to Hardmode after the Wall of Flesh boss is defeated for the first time. Hardmode grants players new challenges and gives them much more content to explore.
Hardmode adds two new biomes to the world that will appear and function upon converting: The Hallow and the Underground Hallow. Along with The Corruption, The Crimson, and their Underground versions, they will spread continuously and at a steady pace in Hardmode. They may eventually overtake the entire world if not dealt with properly.
At this point, Demon and Crimson Altars can be broken with the Pwnhammer, which drops from the Wall of Flesh. Upon doing so, new ores become available to start the player off in Hardmode.
Each time an altar is destroyed, there is a 66% chance that a single random Stone Block in the Cavern layer will be converted to Ebonstone/Crimstone or Pearlstone, facilitating the spread of Underground Hallow or Corruption/Crimson. No block other than the selected one will be converted initially.
Along with the regular enemies, Hardmode contains new enemies that are more difficult, some of which are "upgraded" versions of pre-Hardmode enemies.
Hardmode also makes available new armor sets, weapons, accessories, and other various items that will aid the player and are essential to dealing with new challenges. Some of these become available for purchase from NPCs, drop from enemies, or are craftable using new Hardmode crafting materials and crafting stations.
On the 3DS version version, when the Wall of Flesh is defeated, a pop-up box will briefly appear describing Hardmode while the world is being converted.
