While playing RPG, you will eventually gain access to Hardmode. To unlock it, you must acquire two specific properties:
Once you have both the tools and the Observatory, you can begin finding Sky Pillars and raiding them for loot.
After you begin raiding the Sky Pillar, you will start gathering Aetherium Ore. This is your first introduction to Hardmode materials. With it, you can craft Hardmode tools such as Aetherium pickaxes, axes, and harpoons.
Additionally, when raiding standard biomes such as the Forest, Ocean, and Mines, you will start collecting new Hardmode items like Biome Souls, which represent the next stage of your progression.