Okay, Clear up Path of the Titans for Me?

When I heard Path of the Titans was coming, I... sort of shrugged. I had no idea what it was, but there was an uproar from players saying "OHMAIGOOOODTITAAAAANS" like it was an old friend or something.

Now, I don't know anything about what it was, what it was linked to, or how it worked or what it did.

Can someone please give me one, definitive statement on whatever vague idea was behind it?