Pre-Hardmode is the initial state of the player's world. It is essentially an 'easy' version of the world, featuring enemies and items appropriate for new characters. Pre-Hardmode begins at world creation and ends with the defeat of the Wall of Flesh. Defeating the Wall of Flesh will irreversibly turn the world to Hardmode which adds more content and makes the game more difficult. This is part of systematic game advancement.
The term "Pre-Hardmode" refers both to the Pre-Hardmode world itself and to content accessible in Pre-Hardmode worlds.
There is little reason to keep a world Pre-Hardmode as Hardmode does not remove Pre-Hardmode content (with a few exceptions). Reasons for postponing Hardmode include having an easier game experience, or to make extra preparations (such as building Houses secure against enemies, or digging tunnels to protect biomes) before Hardmode begins.
Pre-Hardmode features vs. Hardmode features
Some items and enemies are not available in Hardmode, or lose a source of attainability.
|Giant Wormnaturally spawns Underground||Diggerspawns more frequently than Giant Worm Underground ()|
replaces Giant Worm Underground ()
|Blue Jellyfishnaturally spawns Underground and in Cavern||Green Jellyfishspawns more frequently than Blue Jellyfish Underground and in Caverns ()|
replaces Blue Jellyfish Underground and in Caverns ()
note: Blue Jellyfish are still spawnable using a Jellyfish Statue
|Blue Jellyfish Banner||Green Jellyfish Banner ()|
|Cave Batnaturally spawns Underground||Giant Batspawns more frequently than Cave Bat Underground ()|
replaces Cave Bat Underground ()
|Cave Bat Banner||Giant Bat Banner ()|
|Wall Creepernaturally spawns in Spider Caves||Black Reclusespawns more frequently than Wall Creeper in Spider Caves ()|
replaces Wall Creeper in Spider Caves ()
|Wall Creeper Banner||Black Recluse Banner ()|
|Meteor Headsdrop loot||Meteor Headswon't drop loot ()|
|Meteor Head Banner||Meteor Head Bannerare considered loot and won't drop ()|
|Shurikenfrom Pots||Grenadefrom Pots.|
note: Shurikens can be obtained from other sources.
|Silver Bullet(from Arms Dealer)
only during Blood Moon
|Silver Bullet(from Arms Dealer)|
|Cascadedropped by any enemy in or near The Underworld||Hel-Firewill drop instead of Cascade|
|Gray Zapinator(from Traveling Merchant)||Orange Zapinatoris sold instead of Gray Zapinator|
|Pre-Hardmode Cratescan be caught while fishing ()||Hardmode Cratescan be caught while fishing, replacing pre-Hardmode Crates ()|
As mentioned, these features (aside from the exceptions shown above) are still available in Hardmode.