Before communicating with Lion, Guilliman had learned from the Departmento Munitorum's report about the transport fleet.
The Departmento Munitorum had been established by Guilliman himself after his return. It was responsible only for tallying and integrating supplies aided by the Imperium and Talon. It possessed few ships, so most transportation of supplies was handled by other Imperial institutions.
Upon learning of this, Guilliman did not immediately place all the blame on the High Lords.
After all, inefficiency and mistakes within Imperial institutions were nothing new. If prioritizing the transportation of supplies to Orphus, beyond the sector and directly on the battlefield, according to the rules, was an error deserving punishment, then such errors had accumulated to the point where heads would roll from top to bottom across the Imperium.
But the appearance of the Human Emperor and the Grand Master of the Assassinorum, a High Lord, confirmed Guilliman's suspicions. This had been done by those High Lords secretly opposing him.
The Human Emperor had not remained with the Ultramarines fleet from beginning to end. He had left Talon for a time and only reappeared recently.
He had appeared in the Grand Master of the Assassinorum's private residence, directly using Psychic Power to persuade the Grand Master to leave Terra with him and witness something firsthand.
What he was to witness was that five High Lords had already left Terra and traveled to a Tomb World in the eastern galaxy.
The Grand Master had thought it impossible, for he had personally seen all the High Lords just conclude a meeting. Yet he did not know why the person before him seemed so convincing, so he followed the Emperor, who had not revealed his identity, to the Tomb World.
There, he saw the ceremony the Necrons had prepared to welcome honored guests.