The Ancient Falmer Cuirass is a piece of unique light armor and part of the Ancient Falmer Armor set found in The Elder Scrolls V: Dawnguard. It has the same ratings as glass armor.
Two versions of the cuirass with differences exist, similar to steel armor. The version worn by Knight-Paladin Gelebor doesn't include pauldrons while the one worn by Arch-Curate Vyrthur does.
The cuirass, along with the rest of the set, can only be obtained from the corpse of Arch-Curate Vyrthur after defeating him at the conclusion of "Touching the Sky".
The cuirass can be upgraded with refined moonstone at a workbench and also benefits from the Elven Smithing perk, which doubles the improvement.