Actually there is a bug when you meet and kill an Arthron Umbra (it states “Prepare …” after the kill) and you don’t have the newest DLC “Legacy of the Ancients” (or not activated).
Look here:
… and at least two more threads about the same bug on top of this section.
If this is your problem too, all you can do is wait for the next patch. This bug has been confirmed and a patch was announced two weeks ago, but unfortunately nothing has happened since then.