Sons of Horus

Updated to include the Sons of Horus.

Additional update includes the "Legion" force org type, which has only one entry: Legion Astartes. This allows users to select their legion affiliation through a single entry, which in turn will unlock/unhide their respective entries.

Lords of War and Fortifications do not currently have this attribute, as they will not normally have a legion affiliation. Primarchs are the exception, however it seems too awkward to force users to add another identical legion choice box to the Lords of War detachment, only to choose a single Primarch model. This leaves no restriction on Primarchs from the wrong legion being chosen. Not a big deal, I think. Seems like a pretty obvious choice.