The Scarecrow played a big part in the original Arkham Asylum, but he wasn't around much in the sequels. It looks like he'll be one of the main antagonists in Batman: Arkham Knight however. He's leading a charge of other supervillains to finally kill the caped crusader. I'm not sure how this ties in with the shadowy Arkham Knight character, but with Joker gone it looks like things have just gotten uglier.
I'm struck by how old and tired most of the villains look, as well as the even darker, nastier looking Gotham in this game. Even Poison Ivy looks pretty drug out in this trailer. I guess the game really does earn its M rating, a first for the Arkham series.