Once you defeat the Elder and have all the bronze gear and Black Forest foodstuffs that you could want the next biome is the Swamp. You'll be looking for Swamp Crypts and using the Swamp Key that the Elder dropped to get into them so keep it handy. This will start your progression into the Iron Age. You'll also want to build all the upgrades for your forge and workbench that the Bronze Age unlocks. The Swamps are especially deadly at night, so take care.