One of the key concepts that differentiates this setting from a lot of the other monster hunter settings out there is the idea that Earth is a prison for many evil supernatural entities. Reality is a prison, but we're in the equivalent of the house arrest program. The entities that are part of the maximum security program are constantly finding weaknesses in their own prison cells and try to break out causing many of the standard monster types, but also causing zombie outbreaks, Silent Hill-like corruptions of towns, and far more terrible incarnations of the gods and monsters of myth trying to claw their way into our portion of the prison. Buffy-style hellmouths, rips in the fabric of space and time, mad scientists bending the laws of physics, and Hellraiser-type incidents are examples of weaknesses in various interdimensional jail cells invading our cushy little reality.
And the jailers? You don't know. (Although from an omniscient perspective, they're GONE.) There's no one to run to for help.
So how does this apply to a D&D dungeoncrawl-focused campaign?
- effectively infinite variety in opponents - we can assume that these various nasties can range from entire races of creatures to a single hideously powerful entity trapped in various cells geared to keeping them imprisoned;
- rationale for different level designs in the same dungeon - these prison dimension are being manipulated or changed enough to intersect with aspects of our own -- hence the sudden changes of dungeon decor and design as you go deeper;
- different locales for dungeons - they don't all have to take place underground, these sudden outbreaks can hit towns, castles, and even cities;
- opportunity for different styles of adventures - some adventures can be about clearing out momentary intrusions into our reality from "below", while others can be about sealing off a portal or surviving the sudden changes in the rules of reality long enough to get out of the affected area, still others can be about rescuing a once normal castle that has suddenly ceased communications.
Astute, crafty, and vicious DMs can already see the usefulness of this campaign conceit with regard to breaking the rules of the default reality -- it's another reality bleeding into ours; the rules are different here!
- an intersect with the realm of the Primarch of Swords: your sentient sword just became more powerful, more intelligent, and demands to be treated as an equal member of your adventuring party;
- an intersect with the realm of the Rememberers -- the memory-vores: memorized spells don't go away after casting, but each successive use results in the need for an increasingly difficult saving throw -- if you fail, the next time you want to cast another spell you end up casting the spell you've used the most so far;
- an intersect with the realm of Hunger: you need to eat double the normal amount of food, and successively eating the same type of food reduces the nourishment you get from it. Not so bad for adventurers spending several days in the dungeon, but just think about the monsters that have been trapped in there for months since the change...
Best not to abuse it though. This conceit is best used as a background rationale and a way to tie together various horror inspirations under a single overall concept.