Not to forestall further (in-depth comprehensive) answers, but you are likely thinking of the 'Sea Peoples', and there's been a bunch of discussion of that topic in this subreddit! There's a section of our Frequently Asked Questions page about the Sea People featuring not only a great explanation of where the term 'Sea Peoples' comes from, and why it isn't that mysterious by /u/bentresh with regards to the Philistines and with regards to other groups in a separate reply. Additionally there's not only a good summary of what we know about the phenomenon by /u/kookingpot, but even an AMA with the author of 1177 BC: The Year Civilization Collapsed.