This is it. The dev mirrored gratt

There is also a dev mirrored treasure gratt

back then, yellowlightz won the +115 race there, this map was totally empty while the "normal" upper gratt was full of people pvping/farming. There were not a single room free in the "normal" dungeon.

I guess the GMs are of course aware of their game and that they know there IS a mirrored upper gratt and gratt treasure only accessible by venging or being summonned.