My Ice wizard is on the quest "A Cage of Orbs" and just destroyed the Orbs to free Oskar Crowfriend. The location page states that three Ghastly Slaves appear after destroying the first Orb, and a second group of Ghastly Slaves appears after destroying the second Orb. However, for my wizard these second mob enemies did not appear. The first group was there when I entered, so I defeated them before speaking to Oskar. I destroyed all of the Orbs and now see Blackblade and a Loathsome Creeper.

Since this is not a repeatable step to the quest, I don't know if the creature defeat requirements can be verified unless/until someone else has a wizard with this quest.