Adapted from the serialized graphic novel written by Mike Mignola with Richard Pace, and illustrated by Troy Mixey and Dennis Janke. Published in 2001 by DC Comics.
This will be David Dastmalchian's sixth DC comics role. His previous roles include Thomas Schiff in The Dark Knight, Dwight in the TV series Gotham, Abra Kadabra in The Flash, Polka-Dot Man in The Suicide Squad, and voicing Calendar Man and Penguin in Batman: The Long Halloween.
The story about the Elder Gods/Cult of Ghul is an homage to the works of H.P. Lovecraft. This one is from "Mountains of Madness" and a few other Cthulu mythos works, with Batman lore added.
At 56 minutes, Harvey Dent goes to the doctor, Herbert West. Jeffrey Combs who plays Kirk Langston played Herbert West in HP Lovecraft's Reanamator "1985".
When Bruce Wayne views the map showing the Mausoleum of the Serpents, the map shown appears to be a direct copy of the three rivers in Pittsburgh, Pennsylvania (but with an ocean added to the immediate east). The islands and curvature of the rivers Bruce looks at are exactly the same as the rivers in Pittsburgh.