Masked Duet

If you are going for multiple mask runs these are the order of difficulty in my opinion:
Easiest - [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.
Easy - [item=174083] - causes small void pools to appear behind you as you run and stepping in them will drain sanity.
Medium - [item=174079] - you are now affected by 2 afflixes in every area instead of 1. The difficulty of this one varies from week to week as the affixes and type of vision change.
Harder - [item=174082] - all damage by enemies that drains sanity drains 5 times more sanity.
Hardest - [item=174080] - reduces your Sanity by 50%.

You obtain the masks by doing the following:
[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).