The Lost Cathedral is one of the most beautiful and mysterious locations on Pandora and the subject of much wild speculation among security forces and scientists alike. The location is said to have been a clandestine, fortified Na'vi cavern filled with some of the most intense and beautiful bioluminescent plants on the moon. The exact coordinates are unknown, but it is believed to be located deep within the inhospitable Hanging Gardens region. It only appears when fighting for the RDA.
In 2143, a large geo-survey team set out on foot to find the Lost Cathedral. No one from the team was ever seen again.
Avatar: The Game[]
While deployed to the Hanging Gardens, Able Ryder finally discovers the location of the Lost Cathedral. He journeys there to find the willow tree to obtain a harmonic in order to pinpoint the location of a dormant Tree of Souls. However, he discovers that this is the location of an avatar traitor and Na'vi leader named Dr. Harper as well.
It is in the Lost Cathedral that Able Ryder kills both Dr. Harper's avatar, his true human body, and all of Harper's numerous Na'vi guards. In addition, he manages to obtain the last harmonic from the aforementioned willow tree, allowing RDA scientists to finally pinpoint the exact location of the sought-after Well of Souls, a giant tree that can link to Eywa in a similar fashion to the Tree of Souls.
Source[]
James Cameron's Avatar: The Game - Ingame Pandorapedia