There are certain types of Batman stories that bring out the Bump in the Night and Dark Knight, Dark City is one of them. I once thought that Grant Morrison was out of his mind until his run on Batman started to make sense and this is one of those issues he has read for sure. I am not 100% sure, but I am pretty sure this is the first time Barbathos is present in a Batman comic, Barbathos being the Bat Demon Doctor Hurt tries to summon up. Speaking of Doctor Hurt, he does make a flashback appearance in this comic.
This comic would also bring the feel of magick and the Occult with a very small cameo of John Constantine on page 8, the 7th panel, we see the Batmobile race around the corner and the Green Suited (Crisis on Infinite Earths) John is hit with a rush of air, when Vertigo was not quite an imprint yet John Constantine served as a stalker of the shadows who manipulates in silent for the possible good of mankind.
This comic is mentally twisted and that is what I love about Peter Milligan, he plays the psychological thriller card better then anyone in comics. This comic is full of kidnapped babies, hanging people and other excuses for a bloodbath. Peter Milligan and co are just setting up the clues for The Great Detective.