So, once we got the sniper, we opened up a new encounter on Io. It was the same encounter that spawned when the special mob was killed during the public event, but this one is recommended light level 400. When you complete it, you obtain the catalyst to transform the sniper in to the masterworked version.
Anyways, the masterwork requirement is: Blighted Essence (0%). My understanding is that there are chests hidden around inside the instance that you’ll need to open. I also believe that you can only obtain 36% completion per week, not unlike the Worldline Zero catalyst.
Onward and upward!