It's been known since the first trailer that characters transform in to summons. We seen Joshua transforming in to Phoenix once he went in to a rage after seeing whatever character that was get slaughtered in front of him. The ones that can transform are called the "Dominants" who summon forth an "Eikon".

As for this trailer, I really enjoyed it. Personally I think that the combat looks great, and I'm glad to see that the Stagger system that was introduced in XIII remains in place. This encourages the player to switch up the playstyle rather than just spamming attack. Turn based Final Fantasy is definitely a thing of the past now - the series has definitely evolved. Some might not like that, but it is what it is, and if XV and FFVIIR are anything to go by I'm sure the game will have some type of slow down/tactical mode as an option for players not so confidant with the fast paced mechanics.

I'm intrigued by the 1vs1 summon fights. We pretty much can tell from this trailer that Clive is Ifrit. If the battles shown are pulled from the game then we can see that Clive (Ifrit) has a battle with Joshua (Phoenix). Clive also seems to be up against BTM (Big Tall Man/Titan), and UBL (Unknown Blone Lady/Garuda). I'm assuming you fight these characters head on as humans first but then there's some kind of gauge that fills before the characters transform in to their Eikons? I wonder whether these scenes will be interactive in anyway, with Quick Time Events or ways to increase attack power? Who knows! But I'm intrigued and can't wait to find out more about the game!