Cadwallon Militia
The Cadwallon Militia unfortunately has no card or explanatory as to how to join or how its formed.
This is explained from the Living F.A.Q.
Unaffiliated Cadwallon fighters The fighters of Cadwallon who do not have a Guild in their rank are to be viewed as originally from the Militia.
Militia of Cadwallon and the Guilds Within the framework of an army of Confrontation originating from the one of the guilds of Cadwallon, the fighters of the Militia must be bound to the selected guild. In the absence of a contrary rule and until further notice, the cost of this affiliation is 0 A.P. They then have access to the various “solo” capacities of the selected guild.
Militia of Cadwallon In the current state of the texts, the militia of Cadwallon is not a faction in the strictest sense. However, it will likely become so eventually. Therefore, in the same way that a character of a guild played in another must be renamed, a character of the militia must be renamed when played within a guild.