Quick answer: From Rustblade Pavilion, reach the bell at Outpost Courtyard by taking the stairs, running along two walls, then swinging across on a chain; it is a red-cloth-wrapped wooden cross set among bamboo in Weeping Creek Valley.
The bell stays locked until the enemies around it are dead. The whole route runs on foot from the Rustblade Pavilion bell, with no detours.
Where the Outpost Courtyard bell is
A red-cloth-covered wooden cross marks the spot, positioned on a rise among Weeping Creek Valley’s bamboo. A dry-stone wall stands beside it, and the yard camp lies downhill.
While nearby foes remain alive, no E prompt appears at the cross. A corridor campfire is guarded by two Brutes, while another three enemies occupy the yard camp below. The prompt becomes available once every enemy is defeated.
Activating this bell uncovers additional sections of the Weeping Creek Valley map, raising Map Completion to 21%.
How to reach it from the Rustblade Pavilion bell
Leave the Rustblade Pavilion bell and follow the riverbank. Make the small jump across, then turn left and climb the stone stairs.
Continue to the cliff, where scratch marks cover a rock face. Move into the marked wall and hit Space to run upward along it.
Find the next wall, which has big X-shaped scratches. Wall-run up it the same way.
You now stand on a plateau with a twisted tree. Walk across it to the far edge.
At the ledge, turn toward the hanging chain and angle the camera skyward until its Space prompt appears. Hit Space; Soul will swing over to a corridor and land there.
Pass through the bamboo on your way to the courtyard. In the passage, two Brutes guard a campfire; the yard camp beneath the bell has additional enemies. Should you fall into that yard, climb a short step and wall-run a short distance to get back to the bell.
Take out all foes near the bell, approach the cross, and press E to sound it.
