The darkened doorway leads through into another black passage. The organic flooring is lighter here, and kneeling to investigate you can see a faint dragging trail, as if something had been pulled out.
From up ahead, there is suddenly a faint sound, like the fluttering of birds. It grows louder, and louder, until you are sure there must be flocks and flocks of furious winged creatures. You notice that the passage is growing lighter, and as the noise reaches a crescendo, an incandescent flash fills the corridor with blinding light.
As you raise your hands instinctively to shield your eyes, you hear the sound receding again, at a different pitch, as if it were subject to doppler shift. By the time your sight has returned, the passage is black again, and about thirty seconds has passed.
You advance to investigate.