How can I obtain King Attercop's spirit?

I'm on the quest To Catch a King and I can't seem to obtain King Attercop's spirit. If I kill him and then use the soul decanter item, nothing happens. If I use it first, then he gets a debuff saying that his soul should be captured ... but it isn't, whether I wait or just kill him. The only information I can find about this is someone having the same problem on the Rift forums nearly 5 years ago.