The cover for Legacies of Betrayal is out and it turns out CotE was right. It wasn't Ultramarine blue/gold on the Cypher snippet.
It was Sevatar in this Who's Who of the Horus Heresy villains. Featuring Lucius, Calas Typhon, Argel Tal, Jago Sevatar, Azhek Ahriman, Kharn, Alpharius and Omegon, Maloghurst the Twisted, Cypher and possibly Forrix. All of whom are looking badass.