When I first encountered this compound, I pictured it as a quaint conga line of traditional Japanese bakemono. Now all I can see is
[redacted topical political comment].
hya-.kki.ya.kou OR gyou
Literally: hundred – oni (often translated “demon” or “ogre”) – night – go
Alternately: A procession of monsters in the night; the night swarming with monsters going about their business as if they owned the place. Supposedly, the origins are in stories of a literal parade of beasts carried out on certain summer nights, against which only onmyou (yin-yang) magic could protect a traveler with the bad luck to meet them. Metaphorically, lots of bad people doing lots of bad things without worrying about the consequences.
Notes: This yojijukugo is essentially synonymous with 魑魅魍魎, but trades the explicit spirit names for a more vivid picture of their behavior. And unlike the origin of 魑魅魍魎 in Chinese antiquity, 百鬼夜行 seems to spring from Japanese folklore. It dates back at least to the medieval times, and has been a popular theme in folklore and art.
Both readings of the final character seem relatively common. I like the phonetic qualities of gyou better, but that might be due to having a non-native phonetic intuition.