Dark Apostle
When the Horus Heresy occurred, most Traitor Legions murdered their Chaplains at the onset of rebellion, as they were more loyal to the Emperor than their Primarchs. The Word Bearers are not most legions, as Dark Apostles can attest. The former Chaplains of the Word Bearers were actually the means that Lorgar used to corrupt his legion to Chaos Undivided, meaning that when rebellion came, the Dark Apostles were leading the charge. Dark Apostles thus hold leadership of Hosts, Word Bearer warbands who operate under the authority of the Dark Council, a collection of the oldest Dark Apostles who lead the Word Bearers while Lorgar spends his time thinking about the Chaos Gods.