This helmet used to spawn once per map reset (around each 6 months) on a chest with some gps, where the Dark Shield Quest is now in the Thais Lighthouse. This item is very rare, even though it was illegally duplicated long time ago. There should be around 15 of these helmets existing on Antica. Also one exists on Hiberna, owned by Sazgih. It can be seen in the Pythius The Rotten's treasure room.