How will the series end? That domanda has bugged me for weeks.

Well because this is a merlin origins series I always thought it would end with Arthur becoming King; Excalibur in his hands.
And Morgana Canto Mordred his lullaby as she starts to prepare him for his fight with Arthur in the further future :)

it depends on how close they make it to athurian legend. according to athurian legend at the battles of camlann king arthur was killed da mordred.