The Forbidden Passage is an area in Forbidden Valley where many battalions of Dark Ronin and one Maelstrom Horseman spawn. The Green Dragon pet is available near Forbidden Passage, and players can pass through the Fallen Gate on the far side of Forbidden Passage (so long as they wear a Maelstrom infused Ninja Hood) to travel the path to the Paradox Refinery.
There is a crypt that the horseman spawns from (with, of course, a smashable gargoyle above it), and a ninja messenger NPC by the name of Bill Shido.
Concept art by Richard Tran
The far side (south) of the Forbidden Passage
The Crypt located in the Forbidden Passage
The Nexus Force Plaque at the Forbidden Passage