The passage is a long corridor, with lightning booming through the area. You need the Key to King's Passage before you can access this area.
The hallway is lined with Stone Knight statues, four of which will come to life and attack aggressively. These knights are in the service of Sir Velstadt, the Royal Aegis. When Velstadt left with Vendrick through the King's Passage, the men under his command dutifully remained at their posts, awaiting his return for so long that they eventually turned to stone.
The Looking Glass Knight stands sentinel at the end of the passage. The King’s Passage was a route taken by the bravest who wished to serve the King as loyal warriors, to prove themselves. It is likely that to prove themselves they would face either the Looking Glass Knight in combat or another challenge. Those who failed to win the fight were sacrificed by the Looking Glass Knight.