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"Black fog" (tentative) is the name of a creature which was planned to appear in Resident Evil 4 during its 2002 Castle stage of development, but was eventually cut. The creature was intended to be created using a new virus developed by H.C.F. and responsible for Leon S. Kennedy's infection when both US Agents and H.C.F. had infiltrated Oswell E. Spencer's castle.
Further notes[]
When asked about this version, Kamiya refers to it by the name "Mist Version" (also translated as "Fog version"), referencing this creature.
The enemy was dropped when it was realised that the level of animation needed to make this creature appear realistic was not possible on the GameCube.
By coincidence, Black Fog is similar to the Uroboros creatures introduced in Resident Evil 5. Like Black Fog, it was also faced by the technology problem; this time Capcom had to invent a completely new animation technique to get around it.