This chapter is narrated by Ozpin and is sort of like the World of Remnant episodes on Youtube.
What you are about to hear is unknown to all but a VERY select few. Long ago, even before Huntsmen were ever created, Humanity lived for a time in fear. Massive Grimm, what we now call Leviathans, were more common than they are now, and attacks from them were all too frequent. However, when all hope seemed lost, humanity got its savior, but not in the form we were expecting.
They weren't Huntsmen; in fact, they weren't even human, nor were they faunus. They were something else. They were monsters of godlike size and power; more than enough to match and even surpass the Leviathans. The people treated these creatures as gods, and gave them their names: the Titans. Little did they realize the Titans were truly gods, and they had a purpose: protect humanity from the Leviathans.
The Titans destroyed the Leviathans, almost wiping them out completely. But they also had peaceful interactions with humans and faunus alike, some, those more magically inclined, even taking faunus form to walk among them and show them they were no threat to humanity, acting as the bridge between our world and theirs. Unfortunately, nothing lasts forever, and soon, a rebellion happened within the Titans' own ranks.
The resulting battle shook Remnant to the core as those who fought to protect the world battled those who sought to control it. Eventually. However, the rebels were defeated, and locked away in different prisons that were hidden around the world. After that, the ones who protected us disappeared. For what reason, we don't know.
Since then, the Titans have practically disappeared from all knowledge, with any writings of them long since hidden. In fact, one can only wonder if they would ever return at all…