So far, they have managed to avoid the worst threats that come from wandering the myriad tunnels and caverns, but after days of travel an event befalls them that they will remember forever - if they live through it.
The tunnel before them is lit only by whatever sources of light the party bears. The first sign of trouble is a series of heavy *clacks* that echo across the stone, the direction of the reverberating punctuations impossible to discern.
Having traveled this far in the silence of the Underdark, the companions know that anything making this much noise must either be predator or prey.