Mythril Ore

Every altar you smash seeds less of this tier than the one before it, so the first few swings hand you the biggest patches you will ever see. Break enough to gear up, then stop. Extra altars keep pushing your world evil further across the map and pay you less each time for the trouble. If you come up short, hardmode crates pulled out of any lake will close the gap without touching another altar.

It sits in the cavern layer and looks close enough to iron at a glance that people run past it. Explosives will not open a path through it either, so the bomb tunnel that used to be your fast way down now needs the pickaxe you just upgraded. Selling is the one thing you can stop thinking about: ore and bar come to the same money, so smelt on convenience alone.

Mithril comes from Tolkien, where it is the light, nearly unbreakable metal the dwarves pulled out of Moria. Terraria showed the ore under that original spelling in early promotional material and then quietly swapped a letter, since the estate holds the trademark.