Chroma

The Horus Heresy is a pivotal event in the Warhammer 40k universe, essentially a galaxy-wide civil war that occurred during the early days of the Imperium of Man, where the Emperor's most trusted son, Horus, betrayed him and led a large portion of the Space Marine Legions into rebellion against the Emperor, causing a devastating conflict that ultimately shaped the dystopian future of the Imperium as we know it today; this rebellion was fueled by the corrupting influence of the Chaos Gods, who manipulated Horus' ambition and sense of betrayal to turn him against his father.