Every world starts in Pre-Hardmode this is the first and easiest phase you encounter in your world. In Pre-Hardmode you can defeat 6 Bosses wich include Eye Of Cthulhu, Queen Bee, Eater Of Worlds(Corruption), Brain Of Cthulhu(Crimson), Skeletron and Wall Of Flesh. Defeating Wall Of Flesh will make your world instantly enter Harmode, there is no delay neither a way of cancelling this so be prepared.
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