Horrific Masquerade

Since the other comment is from an early PTR build here are the updated requirements for obtaining the masks:

[item=174080] is obtained from the end chest after completing all 5 objectives in a single Horrific Vision (both corrupted areas, both lost areas and Thrall or Alleria).
[item=174083] is obtained from the end chest after completing the Valley of Wisdom lost area in a Horrific Vision of Orgrimmar with any other Faceless Mask active (you do not need to defeat Thrall).
[item=174082] is obtained from the end chest after completing the Valley of Honor lost area in a Horrific Vision of Orgrimmar with any other Faceless Mask active (you do not need to defeat Thrall).
[item=174079] is obtained from the end chest after completing the Old Town lost area in a Horrific Vision of Stormwind with any other Faceless Mask active (you do not need to defeat Alleria).
[item=174081] is obtained from the end chest after completing the Mage Tower lost area in a Horrific Vision of Stormwind with any other Faceless Mask active (you do not need to defeat Alleria).

[item=174080] - reduces your Sanity by 50%.
[item=174083] - causes small void pools to appear behind you as you run and stepping in them will drain sanity.
[item=174082] - all damage by enemies that drains sanity drains 5 times more sanity.
[item=174079] - you are now affected by 2 afflixes in every area instead of 1 (also called madnesses).
[item=174081] - when you are below 50% sanity you will spawn an aberration every 1-2 minutes that will attack you and keep you in combat until you kill it. At 460 ilvl it has around 180k health.

The first mask you obtain is always the [item=174080] and in my opinion this is actually the hardest mask to run with as it effectively cuts your time in half. The easiest masks to run with are either the [item=174083] from the Orgrimmmar horrific vision or the [item=174081] from the Stormwind horrific vision. When you have either of those I'd recommend using them straight away as these make obtaining the rest of the masks significantly easier.

If you are going for multiple mask runs these are the order of difficulty in my opinion:
Easiest - [item=174081]
Easy - [item=174083]
Medium - [item=174079] (this varies from week to week as the affixes and type of vision change)
Harder - [item=174082]
Hardest - [item=174080]

As a final note, running these in a group is significantly easier than running solo simply because the more people you have the more cc you have and the less you are targeted by specific abilities.

If your cloak lvl is high enough yes, otherwise its the hardest one.

What more do we you need - Nice post :D