Underworld Houses (also known as Hellhouses or Helldungeons) are common buildings located in the Underworld made out of Hellstone bricks & Hellstone walls or Obsidian Bricks & Obsidian Brick Walls. The amount of Hellforges that can be found in a house depends on how big it is. Most of the time you will find 1 to 4 hellforges per house, but some houses are also smaller and contain no forges at all.
Underworld houses are commonly found within the center third of any given map; if you are not seeing any, it can be one indication of your relative position to the edge of the map (for players who do not possess a Compass). Players who use the Dungeon or Underground Jungle to access the Underworld in search of Underworld Houses may need to move back toward the center of the map to encounter any, since these biomes tend to appear toward the map's edges. Contrastingly, Underworld Houses are most easily found by digging straight down from the original spawn, creating a Hellevator which leads to the greatest concentration of them.
There are also usually a large number of pots in these houses. Treasure chests can also be found inside these houses, but it is uncommon. The bottom floors usually contain lava, and higher floors may also be flooded, depending on the location of the house.
Hellstone Walls are not acquireable in any way. Hellstone Bricks, however, can be mined with a pickaxe and used as a building material, however it is still hot to the touch and is not recommended to make a house out of it, unless you always wear an Obsidian Skull.