(This was an idea from either Vin or Raven or BOTH. Ily both so it's all good ^-^ x )
Skulduggery mentions that he only met Ghastly's mother once.
That one time was when he was Lord Vile, when he killed her.
Ghastly's Mother was one of Lord Vile's final victims. It was Ghastly's cry that triggered Skulduggery's return as the primary ego.