Eye Sigil ARG/Legend of Dungeon
Legend of Dungeon is a game developed by wife-and-husband developer Robot Loves Kitty. The game was released on Steam 13 Sep 2013.
The sigil was added in an update 08 Dec 2015.
There is a dwarf in the bottom right corner of the options menu. Clicking it will make the sigil visible in the options menu (even without filling the password correctly).
Puzzle piece is displayed in a room on the 26th floor when using the remote control near the place where the switch that unlocks the secret 27th level is. You have to unlock the sigil in the menu beforehand.
The piece is stored in the game as a texture (map.tex) at a different orientation than how it's displayed in-game (in the linked screenshot it's additionally mirrored vertically by the tool used). The material the texture is bound to is called legend-of-dungeon-map-piece.mat.
The sigil is stored in the game as a texture too (sigil.tex).