Ghaul's coup was a military-led coup d'état against the former Cabal Emperor, Calus. It occurred thousands of years before the Red War, and resulted in Ghaul's ascension to Dominus of the Cabal Empire, as well as the militarization of Cabal culture.
Long before the coup, Calus was crowned Emperor of the Cabal. Early on in his reign, Calus instigated a purge of the Praetorate, the military aristocracy that controlled the Cabal Legions. Calus viewed the Praetorate as corrupt, abusing its power for its own enrichment at the expense of both the Empire's citizens and the Legions. Although Calus's purge was by and large successful, some members of the Praetorate survived. One member in particular, The Consul, began to plot his revenge, and found what he believed was the perfect vessel for it: an orphan named Ghaul. The Consul raised Ghaul into a powerful warlord, hoping to use him as a figurehead in the overthrow of Calus. At the same time, many of Calus's own allies became disenchanted with their Emperor, equating his decadence with folly and weakness.
Calus had a contingency plan in place against just such a crisis, consisting of a small bone that could be used to deliver a certain signal. Besides him, only his daughter Caiatl knew of his plan, but she was part of the conspiracy against him. Thus, when the coup began and Calus raced to his throne to call in his secret arrangement, he found Caiatl there first. Calus begged his daughter for help, but Caiatl crushed the bone and ensured the coup's uninterrupted success. Over the course of the uprising, Calus's own bodyguard, Shayotet, led a contingent of soldiers to plunder Calus's palace on Torobatl and kill his supporters. With The Consul instructing him, Dominus Ghaul seized control of the Empire's government, and his actions were legitimized by the support of key Cabal leaders such as Evocate-General Umun'arath, the Primus of All Legions, Freeborn Otzot, a Psion savant, Aedile Moli Imoli, the Chief of Festivals, and Caiatl, the Princess-Imperial. Those among the Cabal who were loyal to Calus were either cowed or eliminated.
Unceremoniously dethroned after a reign that had lasted centuries, Calus was exiled to The Leviathan and left to wander the cosmos. Meanwhile, the new regime began its transformation of the Cabal Empire into a nation of conquest. Eschewing the title of Emperor, Dominus Ghaul soon solidified his grip on power, surprising the Praetorate who had hoped to use him as a puppet. Ghaul proved far more strong-willed than they had realized, and the Praetorate learned to fear and obey him.