Fallen Angels by Mike Lee

As news of Horus’s treachery spreads, the Great Crusade grinds to a halt as the primarchs and their Legions decide where their loyalty lies – with the Emperor, or the rebel Warmaster Horus. In this sequel to Descent of Angels, the Dark Angels too face a time of testing, both in the stars, and on their home world, Caliban. Arriving in the Gehinnom system, Lion El'Jonson and a force of DarkAngels set about subjugating the system's core worlds which are key to the Warmaster's plans to overthrow the Emperor. Meanwhile, on Caliban, Luther and the rest of the Legion feel abandoned by their primarch. Civil war erupts as the planet strives to break from Imperial rule, and the Dark Angels are thrown into a deadly conflict where all that they know and trust is thrown into doubt.