Beware the Brakendeth

Humanity had extended itself across the galaxy to over a thousand planets. It had discovered peace, and consequently had little need for a military force. It had never encountered an alien species of any sort. It felt safe.

Then everything changed. Eighteen planets were attacked by an aggressive alien fleet that appeared to devour every living thing. AI Central’s calculations confirmed that this was an extinction event, and recommended that Humanity should flee. However, this was not possible for the trillion odd inhabitants.
Admiral Bonner had a plan. However, she was constantly surprised by the events that confronted her. How could you win against such ridiculous odds?
Battles were fought, and eventually, some level of success was achieved, but they had yet to meet The Brakendeth. Who or what were they? Why were most of the galaxy terrified of them?