Tier 16 of the Dawning of an Endless Night Story Quest in The Secret World
For the 16th tier of the Dawning of an Endless Night quest, you need to go to the Wabanaki Holy Site. The tunnel entrance is shown below. On the map, it is where I (Volaric) am standing. Below that is what the tunnel entrance actually looks like. Go through the tunnel and into the holy site itself to watch a few cut-scenes.
The Secret World Dawning of an Endless Night Tier 17 Guide: Here