I'm guessing in addition to Optimus being the "last" knight, the medieval parts of this trailer, along with the Anthony Hopkins monologue, means it's probably about some form of Knights Templar having the "secret" of why transformers keep fighting over Earth, which maybe involves Unicron? Hopkins maybe being the last of the Knights Templar...?
Hopefully the movie also addresses how having a chunk of Cybertron (or whatever it is) impacting the Moon is a "bad thing with actual frigging consequences", unlike how having Cybertron suddenly being in close Earth orbit would be apocolyptic, even for the time it existed in Dark of the Moon, but instead had no effect whatsoever.
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