Iridescent Ooze

Head to the Plague Watch area in Maldraxxus. The Plaguefallen Chest sits atop the dam walls holding the jelly of the Fount of Pestilence, near the Plague Watch wind rider. You cannot simply walk up to it directly.

Go to the Scunner rare area and look underneath the platform for a tunnel leading into the sewers at 62.4, 76.56 in Maldraxxus. Inside the sewers, follow the tunnel to find a pipe at the far end — this pipe is highlighted like a treasure but does not appear on the minimap.

Before using the pipe or opening the chest, you must become Plaguefallen. Stand in the toxic green jelly in the area until you accumulate 10 stacks of Concentrated Plague, which transforms you. While stacking, you take increasing nature damage, so eating a food that restores a high amount of health — such as Seraph Tenders — lets you sustain through all 10 stacks safely.

Once transformed into Plaguefallen, return to the sewers at 62.4, 76.56 and use the pipe at the end of the tunnel. This launches you up to the Plaguefallen Chest on the dam wall. You have 2 minutes to reach and loot the chest, but the debuff reduces your movement speed, so move immediately after using the pipe.

Open the Plaguefallen Chest on top of the dam wall to receive the Iridescent Ooze. When first summoned, the pet visibly changes color — players have observed purple, green, and orange — though it does not continue shifting colors while actively out in the world.