Hastur Sejanus
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.
Unfortunately, a little more than one year later, he was killed when he let slip that he served an Emperor during a diplomatic conference with the delegates of... the Emperor of Mankind. Horus was understandably mildly perturbed that his favorite son was slain at a diplomatic occasion, but shit like this happens.