Cadia is completely destroyed in the 13th Black Crusade, with Abaddon allowing the 2.5 million (out of a starting 850) survivors to flee, to spread word of his victory.
The Fenris System is crippled by the forces of Magnus the Red, who succeeds in fully invading all the inhabited systems, including sacrificing the planet of Midgardia to return Sortiarius (Planet of the Sorcerers) to the Materium, over the ruined planet of Prospero. Also, the populace of Fenris is culled by the Inquisition (with Grimnar’s reluctant approval) due to witnessing a Daemon Primarch. Fun times.
Also, did you know that the Order of Our Martyred Lady is led by Twin Sisters? Nice.