Word Bearers, originally known as the Imperial Heralds. They were one of the nine Legions which betrayed the Emperor during the Horus Heresy, their allegiance pledged to their Primarch Lorgar and to Chaos as a whole. Known for their extreme religious fervour even before their conversion to Chaos, they are some of the most fanatical Chaos Marines, notable for using Dark Apostles (a corrupted version of Space Marine Chaplains) to inspire their Marines and their cultist allies in battle.
Iron Warriors Warsmith. I don't know who the artist is that painted this picture but I love it. It captures the grit and power of a Chaos renegade without all the warp stress you usually see in GW artwork.