Reziarfgs are a type of monster related to, but not strictly from, the computer game Diablo II. They are special in that they are listed on Blizzard's website along with all the other monster types, but do not actually appear in the game. They are an April Fools' joke posted on the website simply for humorous purposes, especially as the name, when reversed, gives 'gfraizer', Geoff Fraizer being the name of Blizzard's website manager.
The reziarfg page is set up slightly differently than the pages for other monster types. Rather than an animated graphic, it shows a still picture, which can be enlarged by clicking on it. It also shows no monster attributes, although it does state that the monster is a demon (monsters in the game are divided into demons, undead and 'animals' which basically includes everything else). In addition, the description of the reziarfg is written as a sort of deliberate exaggeration of the dark, sinister style in which the other monsters are depicted, highlighting its status as a joke.
Despite the original humorous intent of the monster being shown on the website, some Diablo II players have latched onto the idea of the reziarfg as an actual game monster. It is often treated much like the Cow Level, that is to say as something which is just a sort of tongue-in-cheek rumor in Diablo II but which might be implemented as an easter egg in a future version of the game. In these cases it is usually thought of as a very large and powerful monster, and one which would probably drop some kind of good item when killed. Several humorous photoshopped images of players fighting a reziarfg have been made to date.