The Ethereal Horde

The Etheral Horde is all that is known of the cycle ending phenomenon. From what little records can be salvaged of the end of past cycles, the name "Ethereal Horde" is the name that keeps repeating, with no discernable origin. Cycle after cycle refers to them as some force of annihilation right before the end of any further record, presumably responsible for said cycle end.
In reality, and beyond the comprehension of most all in the galaxy, the Ethereal Horde are essentially the spawn of Hell. The endless and unstoppable tide of annihilation is formed of malignant spirits from an otherworldly origin. At some outset, some milestone within the galaxy achieved, or some seal seal broken, the tide is set upon the stars. Seemingly unstoppably, the denizens of Hell start upon the noospheres within the galaxy, snuffing out the intelligent races for their sins.
The frequency of this phenomenon seems irregular, with no set yearly intervals, and so is judged to be a reaction to some stimulus.