If I remember correct, it's actually Resist Miasma +15. It makes it so it takes longer before you take Miasma damage by 0.5s per tick. It's decent in multiplayer if you like to fight outside of the chalice range, but definitely very niche.

>>1 It's a bit counterintuitive because that chest is typically for defense artifacts. Probably why so many people have a hard time finding this one. There is a separate chest dedicated to armor in the first section, but Square Enix had to make it difficult I guess. Crystal Mail is nothing fancy either, +24 def with +1 miasma resistance. How important is fighting off miasma when you already have a crystal chalice? I don't know, that's up to you to decide.

It's hard to find good information about it online, but if you want the Radiant Armor Scroll for the Crystal Mail, you have to go to the second section of the lv3 Lynari Desert and open the chest/cactus on the bottom left most part of the map (you guys have it marked off as chest #4 in the second section).