An Iron Warriors novel

Hell has come to Hydra Cordatus, for a massive force of terrifying Iron Warriors have invaded the planet and lain siege to its mighty imperial citadel.

READ IT BECAUSE
It's one of the true Black Library classics, and one of the first novels to make the bad guys the heroes... and it introduced the mighty Warsmith Honsou, who went on to plague the Ultramarines. It also pits the Iron Warriors against their oldest foes, the Imperial Fists, which is always a guarantee of quality action.