The Iron Legion, led by the Annunaki Archduke Dagon, was dedicated to support and protect humanity from the Host and its influence. Alongside the Ebon Legion, they were also staunch enemies of the Host and fought them regularly, but without the savagery of their brethren.
When Genhinnom was under siege, the Iron Legion held the line alongside the Crimson Legion until the last Bastion fell and Lucifer was bound in chains.