Hardmode

Hardmode is a harder version of the initial world. You can activate Hardmode for your world by defeating the Wall of Flesh.

Hardmode has many tougher challenges and monsters:
 * Right after defeating the Wall of Flesh, as said announce, "Dark and Light spirits will escape from The Underworld". They will leave behind two lines of The Hallow and The Corruption that extending from The Underworld right to the surface.
 * The Corruption and The Hallow will be able to convert stone and sand to their corrupted/hallowed variants. However, they can not convert each other.
 * The Corruption will be able to turn mud to dirt and spread through it.
 * NPCs will sell new items. (The Merchant will sell the Disco Ball, the Dryad will sell Hallowed Seeds, the Arms Dealer will sell the Shotgun, etc.).
 * Lots of existing monsters will be supplemented with harder versions (for example, the Armored Skeleton as addition to the common Skeleton). This does not mean they stop spawning altogether, just more rarely.
 * You can destroy Demon Altars with the Pwnhammer, which will cause the world to be blessed with one of the ores introduced in version 1.1. The first Altar destroyed blesses the world with Cobalt, the second with Mythril and the third with Adamantite. Additional destroyed Altars will repeat the cycle. This can only be done in hardmode, otherwise the Pwnhammer will act like a normal hammer and hurt the player when the Altar is struck.
 * Existing bosses will come back for revenge as The Twins, The Destroyer, and Skeletron Prime (You can still summon and fight the old bosses).