Pharos: The Horus Heresy, Book 34 (Unabridged‪)‬

With the noble Emperor Sanguinius ruling from Macragge, Imperium Secundus stands as a lone beacon of hope even as the Warmaster's forces continue to ravage the rest of the galaxy.

Roboute Guilliman, still Master of Ultramar, has convinced his brother that Terra has fallen and that the mysterious Mount Pharos on Sotha now holds the key to mankind's future. But the Night Lords, those cruel and pitiless sons of Konrad Curze, have been watching from the shadows and make ready to launch their long-planned attack on the Pharos itself....