The Blood Ravens lost 4 entire companies in Kronus, halving their Chapter.
Then they lost a bit in the Tyranid invasion in 2 and dwindled during their civil war with Kyras and the traitors.
By 3 they somewhat replenished their ranks but the Great Rift split them up and they responded to every call turning them into Lamenters tier until the Custodes arrived and gave them Primaris gene seed.
