The Horus Heresy: Volume Three (The Horus Heresy, #11-15)

The opening moves in the great Heresy took place long before the first bombs fell upon Isstvan III. Old grudges and rivalries among the primarchs and their Legions provided fertile ground for the seeds of treachery to be sown - sorcerers, assassins and fanatics face one another on every front, though it will likely be the Ruinous Powers of Chaos that reap the final rewards...

Read it because
The first seeds of heresy grew in bitter soils for some time before the tragedy at Isstvan – read how the great schemer Erebus began his plot in The First Heretic and turned Lorgar from his father. Magnus the Red's Thousand Sons take centre stage in A Thousand Sons and are punished by the wolves of Russ in Prospero Burns. In Fallen Angels the Dark Angels face internal strife while assassins duel across the stars in Nemesis.