The Parasite Chambers Key open Parasite Chambers dungeon.
The Parasite Chambers is a high-level dungeon that was released for Halloween 2017 (October 24, 2017) for Patch X.18.0. It was based off the scrapped Kabam-era dungeon called the Parasite Den. The dungeon is a good source of Potions of Mana and drops a large number of untiered items.
The dungeon has a chance of dropping from Scout Colonies, which spawn in parasite ruin setpieces in the Highlands.