So it is likely that you cannot currently obtain it anymore or at least currently. The changes made to Siptah and the new camps added took the spawn for the boss that dropped it. I have yet to find where he was moved if he was moved. It’s one of the forlorn souls that used to drop it. There were a total of 3 and now there are only 2 that I know where they spawn. They are the glowing 3 skull boss skeletons that are gold. I did a quick look and the 2 that remain from the older version do not drop the bow. I knew I had seen this bow before but it’s been a long time. You can watch this video if you like it’s queued up to the time where I show the Hunter in the Moon bow. Yog's Touch and more on Isle of Siptah Conan Exiles 2020 - YouTube
Sorry I am not bringing good news but if you can find where they moved the 3rd glowing gold skeleton he may be the one that drops it.