15 is Finders Keepers: claim Darth Nihilus' mask.
First, Defeat Nihilus in the engame. If Visas survives the battles with Nihilus, She will ask what you want to do with the body. Tell her to bring you the mask to get this achievement.
The mask isn't an actual item, it won't be in your inventory.
I assume you have a chance to grab the mask if Visas doesn't survive the fight, but I haven't tried it.