In the Isstvan System, far from Terra, under Warmaster Horus's command and urging, his own XVI Legion, the Sons of Horus, along with the 63rd Fleet of the Great Crusade; the fleet led by Angron, Primarch of the World Eaters; the fleet led by Mortarion, Primarch of the Death Guard; and the fleet led by Fulgrim, Horus's close friend and Primarch of the Emperor's Children, finally assembled outside the Isstvan System and completed the merger of their fleets.
Immediately afterward!
In order to completely eliminate the "uncontrollable forces" within their own and allied forces—those incorruptible diehards, namely the loyalist Astartes Space Marines who refused corruption—they carried out large-scale internal troop movements in the final moments before the merged fleet reached the third planet of Isstvan, under the pretext of preparing to suppress a rebellion and positioning the landing forces in advance.
While Ali had no choice but to be transferred together with Loken, the thousand-strong company under her command, reorganized after she began cooperating with Horus, was also transferred in its entirety to the escort ship named the "Fledgling." Officially, it was merely an ordinary Imperial auxiliary regiment, but in truth, it consisted of the core troops of Ali's resistance and their families.
More interestingly, transferred alongside Ali's company was a group of roughly three hundred Marauder squads from Abaddon's First Company and other large companies—about fifty squads in total.
The key point was that they had not been transferred under the name of First Company Astartes Space Marines. Instead, they had removed their MK-series power armor, put on ordinary Imperial Army uniforms, hunched their bodies, mingled among the company of Ali's resistance, and slipped aboard the ship.
Moreover, perhaps to prevent anyone from noticing anything unusual, once they reached the escort ship's barracks, they immediately hid inside individual compartments