Ecto Mist is a visual effect produced by the Graveyardmini-biome. It is translucent and floats along the ground. The mist will appear on top of solid blocks in an unknown radius. Occasional wisps will appear in the air. As more gravestones are placed, the mist gets thicker and the background gets darker.
Ecto Mist can, in conjunction with a crafting station, be utilized to craft certain unique items; most notably blocks and walls that are generated naturally and do not drop as items when destroyed. The player does not need to be in direct contact with the mist for that; being in a Graveyard is sufficient.
Ecto Mist cannot be used to craft the following naturally-generated objects:
Plant blocks (mainly grass, flowers and vines, but also including Plantera's Bulb and thorny bushes) - these can instead regenerate given enough time
It is also worth noting that walls crafted via Ecto Mist are different from their natural counterparts in ID; notably the Spider Wall will not spawn cobweb blocks nor allow for the creation of artificial Spider Cave biomes. It is also not possible to create an artificial Underground Desert biome using crafted sandstone walls, as these also have a separate ID. You can, however, craft Infested Spider Walls and Treacherous Hardened Sand Wall with the use of shimmer. These walls will allow you to create an artificial spider cave and underground desert biomes respectively.
Ecto-mist is the name given to light, swirling gray mist that marks the presence of paranormal activity. This is a visual effect often used in horror movies to give the atmosphere a more ominous feel.