Here is my army background for the upcoming Tarsiss 4 Horus Heresy event:
‘The Prophet Ghore preaches the revelations of Blessed Lorgar to the illiterate and dispossessed. We of the Covenant believe in the sacred Octet: The Four Eternal Primordial Powers and their Four Scions that will come to be by revelation and Apotheosis. Embrace the Prophet’s teachings – for unlike the False Emperor – the Primordial Powers are generous in their gifts of flesh and spirit to the faithful. . .’ Malgrist, mutant disciple of the Covenant of the Octet Pantheon.
The origin of the Covenant is tied to the destruction of the Khurian city of Monarchia by the Ultramarines. While intended as a chastisement of the Word Bearers by the Emperor, this became a festering psychological wound on the previously pious of citizens of Khur.
Malachi Ghore was one of the survivors of Monarchia to have been taken in by the Word Bearers. Revered as a holy martyr, Ghore was nursed back to health and tutored in the revelations from the Book of Lorgar by Chaplain Valsavis of the Flayed Hand (a favoured acolyte of the arch traitor Erebus). Such teachings ignited Ghore’s twisted piety and he soon began spreading his dark enlightenment.
The Covenant participated in the surface battle against the Ultramarine’s human axillary forces of Calth. While many cultists were ultimately spent as in sacrifice as part of Erebus’ ritual to raise the Ruin Storm, the gaze of the Octet looked favourably on Ghore sacrifices as he escaped with the surviving elements of the Covenant to participate in the Shadow Crusade across Ultramar and the pillage of Takaan-Reva.
Now seeking to extend the armoured strength of the Covenant, Ghore has used the archaeotech artefacts stolen from Takaan-Reva to buy the favour of Archmagos Draykavac of the Forgeworld of Cyclothrathe.
So the Covenant now descends upon Tarsiss 4 alongside allied House Atrax Knights gifted by Draykavac.