The Cnidarian Reef Key open Cnidarian Reef dungeon.
The Cnidarian Reef is a dungeon designed by Atrapper. It was on Public Testing before being officially released in Patch X.27.0.0.
This dungeon drops from the Court of Oryx, and is the 4th Court dungeon to be introduced (3rd if you exclude the event-exclusive Ice Tomb). As with the other Court dungeons, consists solely of a boss fight. The Reef is a source of various stat pots (mostly Attack and Defense) and two unique Untiered abilities.
Uniquely, the dungeon has an underwater theme much like the Ocean Trench. Like the Trench before it, players have to watch their oxygen levels unless they want to die of suffocation. However, suffocating is an even larger risk in this dungeon since it drains air much faster than the Ocean Trench, in addition to positioning the air vents in much more perilous locations.