Passage of the Nameless

Passage of the Nameless is a route rather than a place, and that is the whole of the problem with it. It is a corridor through the part of the continent where identity has worn thin from traffic. Things that walk it long enough stop having names, then stop having duties, then stop having the memory that they used to have either. The ones you fight are at various points along that road.
This is the dungeon where Varden's horror turns inward. Everything earlier in the game wanted your gear or your life. The Passage does not want anything, which is worse, because you cannot negotiate with attrition. The stalkers and shadow assassins in the opening tiers were people. The Shadow Reaper and the Dark Executioner have been reduced to a function and perform it flawlessly. Nothing in here can tell you when it crossed over.
Encounters

Tier 1
Nameless Stalker

Tier 2
Shadow Assassin

Tier 3
Void Rogue

Tier 4
Dark Slayer

Tier 5
Udrak (Mini-Boss)

Tier 6
Nightblade

Tier 7
Void Assassin

Tier 8
Shadow Reaper

Tier 9
Dark Executioner

Tier 10
Azh-Rael Pan Otchlani (Final Boss)
The final boss
Azh-Rael Pan Otchlani is the far end of the road, and the only thing in the Passage that still has a name worth using. Whether it kept that name or took it is not a question anyone has been able to put to it. It fights with the terrible economy of something that has discarded everything about itself that was not required for this, and it has had a long time to decide what was required.
Before you enter
The recommended level is 40. Treat it as a starting point: farm earlier tiers, upgrade gear, and adjust your pet when a boss stops you.
Difficulty inside a dungeon comes from one number that climbs with the tier, and the step between tiers is a large fraction of the gap between dungeons. So a wall at tier 7 here is not a signal to grind this dungeon harder - it usually means the opening tiers of the next dungeon are a better use of the hour, and you come back with the gear they dropped.
