Ironfire is a short story in the Horus Heresy series, by Rob Sanders. It was released online in May 2015 and subsequently included in the anthology Eye of Terra.
His body broken by the fall of the Schadenhold, the Iron Warriors warsmith Idriss Krendl is nonetheless far from defeated. Now commanding two of the mighty siege guns stolen from the forge world of Diamat, he seeks to redeem himself in the eyes of his primarch by bringing down the Great Selenic palace on Euphorus, and gather invaluable tactical data for the eventual assault on Terra. His plan? To utilise a strategy of his own devising - the glorious Ironfire protocol.