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