If you kill Yathosil without capturing her soul, Tynachos asks you to bring a soul of another spider daedra matriarch or monarch. However, he only accepts the souls of generic TD creatures and doesn't react to having unique spider daedra souls (such as Shambalu, MV wilderness or Tur Julan ones). Either add new checks for them or just fail the quest if Yathosil's soul is lost.