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Harmony is a Daemon World in the Eye of Terror. The world served as one of the largest bases of the Emperor's Children during the Legion War in the aftermath of the Horus Heresy. In addition to housing large numbers of feuding Dark Mechanicus city-states, Harmony was also home to the Canticle City, the fortress of the Emperor's Children. It was here that Fabius Bile held the cloned Horus Lupercal.
In the end, Harmony was assaulted by forces of the newly created Black Legion under Ezekyle Abaddon. Thousand Sons Sorcerers allied to Abaddon psychically dropped the Cruiser Tlaloc from orbit during the battle, wiping out the Canticle City and forcing the Emperor's Children to flee the planet. After dealing with the Emperor's Children, Abaddon killed the clone of Horus.