They had found information on the Fifth Company in the database of Dol, the Fifth Company Librarian.
After they left, the Blood Raven Chapter's Fifth Company returned to the Blood Raven Chapter's flagship, the All-Knowing Mystery.
But the Blood Raven Chapter's Fifth Company rebelled.
The entire Fifth Company, which had once wandered through the Warp, rebelled after returning.
They opened a rift in the Warp and summoned daemons.
At that time, the Imperial frontier was suffering an invasion by a Tyranid Hive fleet.
With the Tyranid Hive appearing and Chaos taking the opportunity to stir up trouble, the Warp rift torn open by the Fifth Company unleashed a massive number of daemons into the Imperium's inner territories. Fortunately, at the final moment, the Blood Raven Chapter's Chief Librarian advanced and activated a ritual. Through a psychic ritual, he and two other Librarians completely stitched shut the rift the enemy had torn open, preserving the Imperium's frontier.
After learning of the Blood Raven Chapter's rebellion,
Cyrus wanted to return to the Blood Ravens even less.
Although the flagship had suffered some damage, the Blood Raven Chapter would not learn its lesson. Later, in their search for some psychic device, they would continue developing the entirely Librarian-based Fifth Company. Dol was part of that newly formed Fifth Company.
Rather than return and endure the Imperium's suspicions and all the restrictions aboard the flagship,
he would rather stay here and truly accomplish great things.
He was beginning to understand how those traitor Chapters felt.
They had not necessarily betrayed in order to join Chaos.
Perhaps they simply believed they could accomplish great things, or that only outside the Imperium's control could they accomplish great things.
The Black Templars and the Iron Hands were both like that.
They had never followed the Codex Astartes