The recent Doctor Strange movie introduced the Marvel Studios fans to the mystical side of the MCU and alternate dimensions, something that was hinted at in Ant-Man with the Microverse. It looks like DC is trying to stake their claim to the ethereal areas with Justice League Dark. But when playing on this level, the stakes are much higher as the film's director Jay Oliva told Empire:
This is the other dimensional kind of place. The nice thing about Justice League Dark is that we get to explore places that you don't normally explore in Justice League, Batman movie or Green Lantern movies. Normally with those things we're dealing more with science fiction. It's like alien worlds or exotic locations, like a supervillain hideout on the moon or a 20,000 League Under The Sea type of thing.
With Dark and these characters, we explore areas that are more in the H. P. Lovecraft type of world, with Cthulhu and The Exorcist kind of thing, things that, I think I mentioned this earlier, where it's not your life that's at stake, it's your soul. You may die, a parademon might kill you or Darkseid might enslave you, but at some point death will free you of that. With what Constantine and these characters are dealing with, they're dealing with the fact that these demons or these supernatural entities will kill you, but even after killing you, they will torture your soul for eternity. That's pretty heavy, and in some ways, what they are safeguarding us against is something greater than even the Justice League can ever safeguard us against: Our entire being.
The House Of Mystery is kind of like the TARDIS, you know? It's more or less Constantine's base of operations and home as well. He also stores the most dangerous artifacts in there, in the vaults. That's why you have a lot of artifacts that are all around. They're all magical things, and the House Of Mystery exists in its own dimension, so that's why it's a good place for safekeeping some of the most dangerous magical tokens and weapons. Constantine's kind of like the guardian of it all.
The way I see it also is that Constantine is kind of like the supernatural version of Batman, in the sense that Batman uses all these gadgets. Constantine, I feel, is not magical. He can't create magic on his own, at least not to the level of Zatanna. Constantine is right in the middle. He's not totally non-magical, but he has an affinity to magic, which means that he can use magical items as well as maybe he can do just limited, small tricks like create fire. But he can't, like Zatanna, go full nuclear. He can't create portals and all that kind of stuff. We don't really see it too much in the movie, but I feel he uses the items that are in the House Of Mystery in the same way that James Bond uses the gadgets from Q Branch. Throughout the movie he uses quite a few different artifacts; these are all things that I believe were stored in there and he now uses them to protect him or to help him in his investigations.
Like I said, Constantine has an affinity to the supernatural and to magic that he can tap into, and that's why he can talk to Deadman. In the beginning of the movie, Batman can't talk to Deadman, can't even see Deadman, but John, because he has an affinity to that world, can. He sees spirits, he sees demons, he sees all the things that we don't see in our everyday life.