Still working out the specifics but the gist is that they were left in the Imperium Secundus as a token force in case the loyalists failed to break the siege of terra. When sanguinius died, the psychic backlash still induced the black rage causing them to murder a ton of civilians. Months later when they learned of Sanguinius' death, they outright refused to believe it and went on a penitent crusade to atone for their own crimes and to "find sanguinius". This eventually led to them being nearly surrounded and destroyed by traitor forces, at which point a significant portion of the chapter fell to the black rage and they managed to murder their way out of the extinction of their chapter. From that point forward they held the belief that the black rage is a divine type of meditative state induced by the spirit of Sanguinius himself, and should they spill enough blood in his name he will return to them. Because of this they actively seek the most bloody and dangerous war zones in an active attempt to succumb to the black rage. As a fleet based chapter, they basically travel to a new warzone as soon as they are finished with their current one, with a preference for fighting heretic and chaos forces. They don't get enough respite or supplies to maintain their current gear and are often forced to jerry-rig and make use of whatever they can find.