As the Warmaster Horus sets in motion his plans to conquer Terra, a vicious and bloody battle breaks out on the Eastern Fringe. The Night Lords begin a war of terror in the Thramas System and the Dark Angels race to stop them. What follows are some of the most vicious conflicts of the entire heresy, with no quarter given on either side. As the Night Haunter slips into total madness, can Lion El'Jonson defeat his brother, or will Thramas burn in the name of the Warmaster?
Purely from a lore perspective - The Thramas Crusade is an absolute highlight — the conflict between the Night Lords and the Dark Angels is described in great detail and packed with really cool lore moments.
For me, this is exactly the kind of deep, atmospheric background that makes the Horus Heresy so enjoyable.