Quick answer: The Rusted Loong Key is in a chest in Weeping Creek Valley, about 4 m from the Lóng Pillar Overlook bell. Its card does not name a gate; an iron gate by the Weeping Creek Lóng Pillar looks like the match.
Where to get the Rusted Loong Key
Head to Weeping Creek Valley and use the Lóng Pillar Overlook bell as your landmark. The key is in a chest close to that bell, on roughly the same level.
The Weeping Creek Long Pillar is in the same valley, and its spot is covered in the full rundown of Lóng Pillar locations. Everything else worth picking up in the area is listed with the Weeping Creek Valley collectibles.
Rusted Loong Key item card
The card reads:
A key fashioned in the shape of a Loong. The gem that once sat in its eye is long gone, and the golden scales have been stripped away by human hands. Yet beneath the rust, the key still hints at the prosperity of its former owner.
What is sealed away behind the gate it was made to open?
What the Rusted Loong Key opens
The card asks what the gate keeps sealed and never names it. The likely match is a Lóng Pillar iron gate in the same valley, near the Weeping Creek Long Pillar, but that door has not been confirmed on screen. The full keys list has the other locks.
