The Bitter Sons  are a band of Iron Warriors who's sole purpose in training and in warfare is to deconstruct enemy emplacements brick by brick, if necessary. The Siege-Masters have a wide variety of tools to be able to get through and break any walls their enemies may feel safe behind, however no tool is as effective as the Lord of Iron himself, who has yet to be defeated when faced with a Fortress of any kind. Maybe the Solar Lords should give him a run for his money and see if they can bring his streak of victories to an end!