i have seen a lot of people in lore videos and in destinypedia refer to ghaul as emperor of the cabal empire. this is simply not true, Ghaul before his end was the Dominus of the red legion a rank which as far as we know was created by and only held by Ghaul.
in cabal history after the midnight coup and the exile of Calus there was no new emperor and the title still belonged to Calus although he had lost all his power. Right after that Ghaul promoted himself to the rank of dominus. before this the highest rank in the cabal military was the rank of evocate general who commanded all the legions and was also known as the primus of all legions. at the time of calus’s reign ghaul was the primus of the red legion which is the most elite and powerful legion with the exception of the royal guard who became the loyalist regime.
Ghaul was only considered an emperor by his most loyal soldiers such as Primus Ta’aun the citizens of the cabal empire did not deem him worthy of the title emperor since after Calus had found the Nothing and got his mysterious powers most of the empire sided with Calus during the cabal civil war seeing that Dominus Ghaul was no match for Calus’s might.
at the time of the red war only the red legion and its closest legions such as the skyburners were the only ones left supporting Ghauls rule. as we see in the first cabal ghost fragment the ghost narrating says that many of the legionaries were arguing and clashing with each other. and many soldiers no longer followed the ancient marching orders they were given. infact the reason Ghaul comes to the solar system to take the light and the reason he wants to be chosen is to prove himself worthy of properly leading the cabal empire. in the first Ghaul cutscene the consul assures Ghaul that he would take the light and that he would be called emperor. Ghaul wanted the light as it would be the only way he would be powerful enough to defeat Calus and win the civil war it was his last chance to
be worthy.
in the game he only called himself as the dominus of the red legion and not the emperor of the cabal
empire. Ghaul for a few years had almost the same power as an emperor but since the return of Calus he quickly lost most of his power to the point where he had to achieve an almost impossible goal to really be the emperor.