Hardmode

Hardmode is a hardened version of normal world you are used to. It was introduced in 1.1 patch. You get there by defeating the Wall of Flesh in The Underworld.

Hardmode has many tougher challenges and monsters:
 * Your world will be blessed with 3 new ores. (Cobalt, Mythril Ore, and Adamantite Ore).
 * The existing bosses will come back for revenge as Skeletron Prime and The Destroyer.
 * Lots of existing monsters will be replaced by harder counterparts. I.e, Corruptor.
 * The Corruption will be able to spread through almost anything. The only known thing it won't go through is The Hallow.