Formidine Rift was mentioned in a couple of books by Drew Wagar. The directions to it are "take a line from Reorte to Riedquat and keep going".
Acaelus Thorn took it upon himself to find out exactly where it is. I jumped in to try and help in the thread.
https://forums.frontier.co.uk/showthread.php?t=135529
Erimus has taken the available information and integrated it into the Galactic Mapping Project.
https://forums.frontier.co.uk/showthread.php?t=116450
There are hints and a couple of quotes of dire things that have happened in recent history in this area. One quote mentions about so many thargoid ships that they blot out the stars (paraphrasing).
We don't know much for now, but the indications are that "something" will happen in this area at some unspecified time in the future.