Herald, or more properly "Moloch's Herald", is the name of Moloch's owl which carries his orb to raise Moloch from the ground. The BradyGames official Guide for Mortal Kombat: Deadly Alliance, as well as Koffin "MY" Kontent in the same game's Krypt includes a piece of conceptual art describing that the owl flies around the world carrying Moloch's chained orb, which contains the Oni's essence. Eventually, as legend has it, that when the owl drops the orb and would let it fall and Moloch is raised from the ground that it touches, starting a rampage on that land. Once his time is complete, he disappears back into the ground. Herald then collects the orb and flies away, starting the circle anew.
Due to the concept conflicting with Moloch's official profile, it could be considered a scrapped concept/idea.