The entrance to the Sanctum is undoubtedly connected to the stone platform near which the Heralds, their message sent, come to die. What exactly this connection is, or how the Sanctum is hidden, no one really knows at this point; not even the Heralds themselves.
The Sanctum itself is a wide valley, ringed by mountains on all sides. Colossal winds prevent flight over them by developing Heralds. It is highly unlikely that even fully grown Heralds would have success without untold damage to their wings, thus costing them their lives.
Within the valley is a deep lake, vivid blue and cold even in summer. An ancient forest runs all the way down to the water on one side, the space beneath the canopy open enough to accommodate the Heralds’ wide wingspans.
The only remnants of anything civilized are a tumble of stones where one wall partially stands still, though only shoulder high. Stubborn weeds grow up through the rubble. It appears that the secret to saving dying Heralds has been forgotten for a long, long time.
The weather in the Sanctum changes with chameleon fluidity. One day snow, the next a gentle rain...what will it be today? Only those here will know.
***
Within this place you will find the Sanctum, yes; but here also you will find the scattered tomes that chronicle the hopes, fears and lives of these angelic messengers who have been reborn.