This RPG never really existed. Fantasy Flight Games announced it, with several updates, in order to draw attention to the publication of Warhammer Fantasy Roleplay (3rd Edition). It is a short story (in four parts) by Graham McNeill.
From the publisher's website:
Perhaps you've heard the rumors... the wild speculation. But you think you know the truth...
In the darkened alleyways and in the backs of sparsely patronized taverns, Altdorf's beggars, deviants, and other assorted outcasts exchange the latest hearsay in hushed tones. In their nightly drunken gatherings, they whisper to each other, embellishing second-hand stories. There have been strange goings-on, they say, beneath their very feet, in the town's sewers. This vast network of dank and foul-smelling tunnels, frequented only by the lowest of the low and usually overlooked by the good people of Altdorf, has recently become the place of exaggerated barroom legend, and far-fetched tales have been flowing through the town's streets like so much excrement through its sewers.