Warhammer 40k - Iron Hands by Jonathan Green

With Abaddon the Despoiler’s Thirteenth Black Crusade engulfing the galaxy, Iron-Father Gdolkin and his company are pulled away from the defence of Medusa to honour an ancient oath to the Adeptus Mechanicus. A trail of clues leads Gdolkin to a long-lost world, home to an ancient weapon that might turn the tide of war in the Imperium’s favour. But for Gdolkin, the quest is personal, as this mysterious planet might also contain the answer to the oldest mystery of the Iron Hands Chapter... the resting place of the Primarch Ferrus Manus.