If a friendly unit with the Mark of Khorne is within 3" of Vorgaroth and needs to take a Break test, you can say that Vorgaroth demands unwavering obedience. If you do so, the unit suffers D3 Wounds worth of models, but the unit will count as being Unbreakable for the rest of this phase. If you do not do so, and the Break test for that unit is failed, it suffers D6 Wounds worth of models (after the opponent has decided to pursue or not, but before rolling the flee distance).