If we know the demons, and we do because we've studied them extensively, they're fast at work repairing any damage that has been done to their precious transporter.
So, if you want a chance at another darkrune, take our banishing crystal back out there and eliminate more demons. Besides, you want to help us to lessen the threat that the demons pose, yes?
We said "chance" because the process we use yields mixed results. You might get a darkrune, or you might get something else entirely.
A seasoned pro such as yourself shouldn't be having any difficulty banishing demons. You're not having any difficulties, right, <Race>?
Completion
We wish the process of creating a darkrune was foolproof, <Name>. Alas, it is not. Please, take this bag and look in it to see what we've wrought today. We couldn't bear to look ourselves.
Remember, if you get a darkrune, take it to Gahk to exchange for a crystalforged darkrune. Then you'll be able to once more bring down Shartuul's transporter.
Oh, and <Name>, don't forget to come by again tomorrow and pickup the banishing crystal. There are always more demons to be banished.