I would think instead of a night's watch agenda why not this:
The Wall
5 gold.
No attachments. Immune to triggered effects. Cannot be discarded.
Instead of having a setup you may place the wall in play.
When you lose a challenge, discard 1 power from the wall.
You may not place or claim power on your house card.
When you gain control of a night's watch character place one power on the wall. When a night's watch character leaves play discard one power from the wall.
Take the Black
Attachment - condition.
0 gold.
Take control of attached character.
Attached character loses all icons, printed traits and gains the night's watch trait.