Abaddon
Abaddon the Despoiler is Horus' successor and is renowned as the single greatest threat to the Imperium in the galaxy. This says more about the Imperium than Abaddon as his raging incompetence is now well known. He has launched thirteen consecutive Black Crusades against the Imperium, every one of which has failed miserably. He will not revise his strategy, so don't ask.
He also has a model in the tabletop game. After you watch this model go down in a single round to a couple of trigger-happy Tau Broadsides, it won't be fun anymore. Fucking railguns.
