The power of the Shadow Guard was underestimated by the dark ones. They had not expected to ever be conquered. They were wrong.   

Although the presence of evil had not been totally eradicated from the face of the world, the amount of evil that was faced in times passed had been greatly reduced thanks to the efforts of the Shadow Guard. 

And because of the depletion of the dark forces, did the innumerable members of the Shadow Guard  begin to dwindle away, until there were none.  Thus, the Shadow Guard became a thing of history and legend.