Demonite Ore can only be found within Dungeons, specifically in The Mines, accessed by finding a Foreman's Key somewhere within the mazelike structure and using it to unlock a Spatial Distortion. It can be used to make Hellforged Ingots or Hellforged Sand.
When mined with Silk Touch, the ore can be harvested directly, but otherwise, it will drop clumps of Raw Demonite.
several decorative variants of this block exist, being faintly tainted with Corrosive, Destructive, Steadfast, or Vengeful Will in a stonecutter.
They can each be directly crafted back into 9 Hellforged Ingots.