Kristos

Kristos took Clan Raukaan after the Skarvus Ambush and won Dawnbreak and Columnus by treating allies as expendable mass. The Raven Guard died on his arithmetic. He called it logic. The Iron Council called a two-century inquiry that split the Chapter into Kristosians and everyone with a remaining doubt.

The Gaudinian Heresy is where the maths showed its working. Chaos had been in the logic the whole time. Kristos went from Iron Father to a problem the Chapter had to burn out of itself.

Medusa still argues about whether the flesh was the weakness or the certainty. Kristos is the exhibit they would rather not display.