After getting both masks you can fight either Faelin or Faerin. I previously fought Faerin • Remnant 2 - Faerin Bos... , which was a fairly difficult fight with okay rewards (he gives the sword I never use). Faelin we found slightly more difficult, though about the same as long time.
After beating him you can get a decent gun, and if you kill both of them their items can be merged into a new amulet. I strongly suggest doing this if you plan to play extensively since you can't hold multiple items.
00:09 - Dialogue Start
03:10 - Fight
05:48 - Faerin Conversation
06:55 - Rewards