Any unit of Undead accompanied by a model with this rule uses the character's unmodified Weapon Skill in place of its own (use the highest Weapon Skill if the unit is joined by several characters with this rule).
If all characters with the My Will Be Done rule in the unit are killed, the unit immediately reverts to using its own Weapon Skill. This special rule has no effect on mounts or any other characters except Tomb Heralds – these always use their own Weapon Skill.