To answer the most important question that surrounds any CGI trailer for the Assassin's Creed series: the song is The Golden Age by Woodkid. As for the video, it—via the medium of acrobatics and violence—introduces a new character: Elise. Described jointly as "an independent young noblewoman" and a "fiery Templar", she's rescued by Arno, Assassin's Creed Unity's lead character, for purposes unknown .
"Watch Arno in this new cinematic trailer," suggests the trailer's description, "while he is in race against the clock to save a Templar! Who is she? How would our new Master Assassin manage his cornelian dilemma?" Another question: is her reveal a response to the controversy and mixed messages surrounding ACU's lack of women as playable characters ? And are you really going with "fiery"?
Other than Elise, there's not too much to be learned: it's a pretty unrepresentative—and unrepresentatively pretty—slice of the game's world. And let's not forget that this is a world we've already seen through actual game footage .