The Mournival was an unofficial council of four favoured officers who stood closest to Horus Lupercal, offering him blunt counsel that no formal rank could compel. Drawn from the captains of the Luna Wolves — and later the Sons of Horus — its members spoke to their primarch as brothers rather than subordinates, each voicing a different temper so that no single view might blind the Warmaster to folly.
By tradition the Mournival numbered four, its ranks filled by acclamation and sealed in a rite beneath the light of a moon, a blade drawn across the brow to mark the new brother. In the years before the war its seats were held by Ezekyle Abaddon, Tarik Torgaddon, Horus Aximand, and Hastur Sejanus, until Sejanus's death opened a place that fell to Garviel Loken.
The Mournival's intimacy made it a lever of ruin. When Horus turned, the corruption of his inner circle became the corruption of the Legion itself, for the brothers who could not be swayed were murdered or cast out while the willing rose higher. What had been conceived as a safeguard against a commander's isolation instead helped bind the Warmaster ever tighter to the powers that unmade him.