I'm putting together a very horrory Dark Heresy adventure. It's set on the Feudal World home planet of one of the PCs; they're called to investigate the fortress of the PC's order of local witch hunters, which has gone dark. The black ships were delayed and their dungeons are overcrowded with increasingly agitated untrained psykers, so there's much cause for concern...
Naturally, everything's gone to shit once they get there. Essentially, one of the masters of the order got corrupted by Nurgle after deciding that their efforts were doomed and he might as well surrender to apathy. The master freed the psykers and allowed them to be possessed, after which they summoned a daemon of Nurgle into the fortress and killed almost everyone else. The fortress gets increasingly creepier and stranger as the PCs delve deeper, with the thinness of the Veil and the accumulated psychic energies causing all kinds of bizarre phenomena.
So, what kind of phenomena and dangers would be cool to work in? Keep in mind the Nurglish theme - decay, entropy, disease. The daemon is 'the worm that eats at the skin of time', essentially a Greater Daemon that's fortunately not strong enough to manifest fully yet. What I'm going with so far is:
-I'll be playing a low heart beat sound throughout. I'll gradually increase it in volume as we get deeper in. The players will notice eventually, I'm sure.
-Early on, the signs are mostly minor. The main gates collapse into rust at the touch. The statue of the Emperor in the entrance hall looks pained and cracked. Food is spoiled, there are clouds of flies, vermin skitters in the shadows. Metal is rusted, wood mildewy, stone showing cracks and crumbling. It looks at parts like the decay of years rather than mere months.
-Mid-way in, we find bodies, evidence of a desperate battle. The rot has set in. Maggots crawl in their multitudes over decaying flesh, creeping out of nostrils and the corner of unseeing eyes, bulging under skin and moving in such numbers they make a perceptible scuttling sound. You can't walk without stepping into streams of insects and maggots. The bodies, if anyone cares to inspect, are curiously withered and bloodless. The only thing they avoid is a statue of the God-Emperor, and only just.
-Forwards, there's a scratching in the walls here. Flickers of movement in the gaps between stones. The masters of the order are largely dead in their beds, half-eaten by rats which are in turn crawling with maggots even as they still live. Icons of faith are cracked and broken. At the end of this first level, liquid begins to trickle out of a wall, growing steadily into a stream, then a flood. It's blood, naturally, and it threatens to knock over the PCs and drown them. There should be probably be nasty consequences if someone ends up ingesting said blood. Admittedly, I'm not sure if a wave of blood fits the rest of the theme, but I like the image of it.
-They have to venture down into the Pit, the dungeons where they kept their captive psykers. They have to get a key they need to advance (or something). Vermin appear to be heading towards one location, which proves to be a massive pile of rats, worms and flies that also holds a body with the key. Naturally, they have to dig into that pile to get the key. After they take it, the daemon stirs. I have a nice frenzied heartbeat clip for that moment. They have to get the hell out of dodge with a massive daemonic centipede pursuing them, trying to block their way with its seemingly endless body. It avoids light or maybe icons of faith - anyway, they have a way to keep it at bay, but it will try to get behind them and block all exits. They get free by getting back into light or to a statue of the Emperor that it can't cross, or something.
-Finally, they go into the upper level. With the daemon active, shit gets even stranger. Time itself begins to decay and fray. With every beat of the heart, rooms change - statues change from pristine into cracked, cracked into crumbling, crumbling into dust; mosaics of the order's heroic scenes have their figures turn older and withered until there is only a field of dust with the Worm twisting through it; walls and ceilings age and crumble, revealing glimpses of alien skies. The implication being that the world will be turned into a Daemon World if they fail to stop the psykers.
So, any ideas for further spooky things? Thoughts on if some of these are unworkable or run the risk of not being even remotely spooky? I'm trying to do a whole 'increasing tension, creeping horror' thing with things gradually turning worse. I thought about things like rooms that lead into themselves, lights going out and coming back on with the room totally different, faces and identities blurring together, all kinds of things. I'm not sure they fit the Nurglish theme of it, though. It might be better to stick to death, decay, entropy, and all that.