Vratine Armour is a special type of Power Armour worn by the Sisters of Silence. When a Sister-aspirant takes her final oaths of duty and says the last words she will ever utter aloud in her lifetime, she is given her panoply of war: the Vratine Armour -- literally "the armour of the oath."
It is a subpattern of technologically-advanced battlegear which shares design elements with both the Power Armour of the ancient Legiones Astartes and the silicate-mesh of Selenite void-mail, though unlike either of those famed armours, it is not -- at least without additional gearing -- fully environmentally sealed or designed for extended operations.
It does, however, provide formidable protection from ballistic impacts and directed energy weapons, and is tailored and forged uniquely to its intended wearer to allow for their complete freedom of movement and to augment their reaction speed.
- The Horus Heresy Book Seven: Inferno (Forge World Series) by Alan Bligh, pg. 251