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At the start of the [[Horus Heresy]], Warmaster [[Horus]] apparently only had three other Primarchs and their respective Legions at his side. Under his command were his own [[Sons of Horus]], joined by [[Angron]] and his [[World Eaters]], [[Fulgrim]] with his [[Emperor's Children]], and [[Mortarion]] and the [[Death Guard]]. After ridding himself of all suspected Loyalist members inside the three Legions at [[Isstvan III]], he choose ''Isstvan V'' as his command post and prepared a deadly trap for his former brothers and their Legions.