The ritual of Drakken-Asca is but one of many rituals of divination and augury proper to the Promethean Cult as it was first practiced by the members of the Salamanders Legion and is still used by the Salamanders Chapter of the Adeptus Astartes today. While the ritual of Drakken-Asca seems to have been adopted from the superstituous tribes of Proximal Secundus, once a recruiting world for the XVIII Legion, the ritual has now been fully integrated into some sects of the Promethean Cult on Nocturne where it is known as the "Dragon's Ash Ritual."
When the Primarch Vulkan wrote the tenets of the Promethean Cult, he included many elements of his homeworld's culture, including obscure practices that were incompatible with the tenets of the Imperial Truth. For instance, some tribes of Nocturne's northern polar regions still made use of the Dragon's Ash Ritual. In ancient Nocturnean dialect, Drakken-Asca can be translated as "reading the Dragon-bones" and while Vulkan himself frowned upon this practice, he never forbade it or purged it from the tenets of the Promethean Cult. As such, the Drakken-Asca was almost exclusively conducted by the Legion's cadre of Igniax, which was the XVIII Legion's term for their Chaplains before the implementation of the Chaplain Edict at the Council of Nikaea.
The ritual involves the inscribing of questions in the archaic Nocturnean tongue upon the bones of a Salamander drake -- the totemic creature that has given the XVIII Legion its name -- and casting them into the heart of a forge. The pattern of discolouration of the dragon-bones in the fire is then supposed to deliver clues as to the events yet to pass.
Known practitioners of the Drakken-Asca include Lieutenant-Chaplain Xiaphas Jurr who is said to have made use of the ritual to safely guide the Ebon Drake in its voyage and locate the stricken form of Cassian Vaughn, which rested under the vitrified sands of the Urgall Depression. It is generally believed that the Drakken-Asca also became a prominent ritual in the beliefs of the Disciples of the Flames, the schismatic group of Shattered Legion Astartes, mostly Salamanders, who were once the crew of the Ebon Drake. The practice of Drakken-Asca has always skirted the edge of Imperial doctrine but became increasingly accepted as the theistic teachings of the Imperial Creed replaced the atheistic Imperial Truth as the defining faith of life in the Imperium of Man.
- The Horus Heresy - Book Six: Retribution (Forge World Series) by Alan Bligh, pg. 36