Once a captain among the Sons of Horus. His father is said to have loved him with a fatherly passion because asides from being an expert in diplomacy, command and martial skills, he was actually a nice and noble person. Not a wonder he celebrated with Horus in the Ullanor Triumph as one of his senior counsellors.
Sejanus was one of the earliest members of the Mournival - enough that he was a member back when Abaddon was still a ganger on Cthonia, and Sejanus and Syrakul "interviewed" him to make sure he'd fit.
Unfortunately, a little more than one year later, he was killed when he let slip that he served the Emperor of Mankind during a diplomatic conference with the delegates of... a guy who said he was the Emperor of Mankind. Horus was understandably mildly perturbed that his favorite son was slain at a diplomatic occasion by the order of someone who claimed his father wasn't the real Emperor, and his anguish over the matter would later be used by the Chaos Gods to corrupt him.