Vial of Forsaken Sicket Waters
Well I suspect the intent is that the item is only able to be acquired after killing him so that only people who kill him can get the item instead of ferrying people in by re opening the instance so that anyone could get it. Obviously that would greatly trivialize the collection.
As already mentioned, you still need the drop from the chest. So cheesing the water would not trivialize the collection. If it’s intended, it would have been nice to know it before.