Once you’ve played Terraria for a while (and have beaten the Wall of Flesh) you’ll enter hardmode, which makes everything… harder. After defeating the Wall of Flesh (WoF) you get rewarded with several items, one of which will always be the ‘pwnhammer’.
With this hammer you can destroy demons altars, which, after being destroyed, trigger a message to pop up on screen. This message will either read: “Your world has been blessed with Cobalt”, or “Your world has been blessed with Palladium. After smashing a second altar, another message will pop up, either reading: “Your world has been blessed with Mythril”, or “Your world has been blessed with Orichalcum”. The third time you smash an altar, the message will either read: “Your world has been blessed with Adamantine”, or “Your world has been blessed with Titanium”. After that it just repeats the order, only spawning them in smaller quantities.
To mine each of these, you’ll need a Molten Pickaxe, which you can get by using Hellstone bars. You can use the Molten Pickaxe to mine Cobalt/Palladium ore, which you can then use to make either a drill or a pickaxe. These new tools can mine Mythril/Orichalcum.
With these new tools under your belt, you can travel down to The Underworld to get the final few ores.
Once you have collected a plentiful amount of Titanium/Adamantite, teleport back up to the surface and craft a new forge from 25 pieces of whatever new ore you just got. With this new forge, you can finally craft new pickaxes, drills, and all the tools you’ll ever need!