You have information. You know what lies beneath the Valley’s floor and it is something that can change the chances of your army’s success. The exact nature of the treasure makes it something critical to deny to your foes, if not directly benefit you. Of course you came here to retrieve it. Discretely of course- no reason to rouse the rest of the Imperial Forces in the system when a neat little civil war would provide enough general chaos to get you what you want. From there you could wage into the rest of the system once the Artifact was yours. However someone else has clearly came up with the same idea and they are foolish enough to stand in your way….