Third Covenant, The

The Third Covenant was a Blackshield group that operated across several sectors to the galactic north of the Great Core, into which they ultimately vanished towards the end of the Horus Heresy. Several studies had been made of the Third Covenant in an effort to identify its members, but most had proven unsuccessful as each Blackshield went to great lengths to conceal their origins.

History
Known for operating in the galactic north of the Great Core, the Third Covenant was especially secretive of its origins. They even renounced their own given names and used instead terms derived from other, more esoteric sources. The group is of particular interest to later generations of Imperial agents because, having rejected the teachings of their own gene-sires (Traitor and Loyalist alike), they turned to other, more shrouded doctrines. Ostensibly, the Third Covenant fought for the Warmaster's cause, but in truth it pursued its own goals often only tangentially co-aligned with those of Horus and his allies.

Warband Colours
Blackshields of this group wore a typically heterogeneous mix of elements taken from defeated foes, often painted over with black. In places the ubiquitous black heraldry would be worn away to reveal the colours of the original owner.

Warband Badge
Of particular note was the mythological hippocampus worn upon the right shoulder as the icon of the Third Covenant. The many esoteric sigils betrayed the Covenant's true allegiances -- to powers other than Terra or even the Warmaster.

Source

 * The Horus Heresy - Book Six: Retribution  by Alan Bligh,  18-19, 218-228, 234, 246-247