Nobody Wants to End Up as Next Week's Fashion
Many in Westeros regard those in the Free Folk army as savages, & many in the Free Folk seem to go out of their way to enhance that feeling. The Followers of Bone are devoted to the Lord of Bones & have bedecked themselves with an assortment of macabre skeletal accessories, including using them as weapons in battle. Their disquieting practices & appearance has a marked effect on the enemy.
The Followers of Bone unit box gives Free Folk commanders a scary new unit to fill their ranks. The horrific visages of these warriors can cause enemies to shrink back instead of surging forward, making them take a Panic Test before they can attack the Followers. Add to that the Champion of Bone Unit Attachment that grants restored Wounds to the unit whenever an enemy fails a Panic Test when engaged with them, & the Followers can be a very durable unit, indeed.