If either Arahan or Naestra is slain, do not remove the model from play unless the other sister is killed in the same phase. If at least one sister has at least one Wound remaining at the end of a phase, both are restored to their starting number of Wounds - otherwise, remove both models as casualties. If either Naestra or Arahan issues or accepts a challenge, the other can attack (and be attacked) as part of the challenge. Should their mount be slain, the sisters automatically form a unit. They can join other units, but if one of the twins joins a unit, so must the other. If one twin cannot join the unit, neither can the other.