Defeat the betrayers again within three and a half minutes.
Souls is obtained by defeating Devola and Popola again within three and a half minutes. Hit either Devola or Popola with Dark Hand and melee them before they teleport, rinse-and-repeat until a cutscene plays. When the cutscene ends, Popola will be the only enemy remaining. She will have very little remaining health, but will be firing a barrage of magic at you. Hit her a few times with your magic or weapon to defeat her.