Chaplain Decimus
Decimus is a relatively recent addition to the ranks of the 10th, having arrived aboard the Final Word.
He is unique in that he is the sole surviving officer of the Guardians of the Word, the original chapter assigned to the Verbum. He alone of the officers had seen the Light, and was thus spared the purges. Indeed, there are some that say he knew what was happening all along, but did not intervene.
His story goes back further, however, for as an officer he was drafted from another Astartes Chapter - the Raptors - where he served, apparently loyally, as a Chaplin for many years.
What prompted his shift in allegiance to Chaos is not know. Some of his fellow Chaplains question his purity and loyalty, but since the Apostle has seen fit to elevate him to Chaplain they wisely keep their peace. It also helps that he's been warmly welcomed by his company, the 18th, and its captain, Guerto "Iconclast" Bahaus.