Mythril Ore is an ore that appears in the world every two out of three Demon Altars or Crimson Altars (Note that its equivalent, orichalcum ore may also spawn and that is the same power.) that have been destroyed in the cycle in which hard-mode ores are spawned. Demon or Crimson Altars can be destroyed only after the Wall of Flesh has been defeated and by using a Pwnhammer or greater(Hammush, The Axe, etc.). Doing this without defeating it first will result in the player's health divided in half each attempt.
Mythril Ore is colored grayish-green and can be found all throughout the Cavern layer. It cannot be mined with Molten Pickaxes. This mineral can only be mined by a Cobalt Drill or higher. To craft all Mythril equipment, the player needs to acquire 180 bars, or a total of 720 ore.