2-Piece

Hellcaller 

Mark of Xavius increases  Blackened Soul damage by an additional 6% per stack.  Malevolence increases the stack count of  Wither by an additional 2 stacks.

Soul Harvester

Shadow of Death unleashes your  Demonic Soul to assault your current target for 12 sec, dealing (232.755% of Spell power) Shadow damage and (155.174% of Spell power) Shadow damage to enemies within 10 yds with each swipe.

4-Piece

Soul Harvester Affliction Single Target

Soul Harvester Affliction AOE Mythic +

Hellcaller Affliction Single Target

Hellcaller Affliction AOE Mythic +

Intellect>Critical Strike>Mastery>Haste>Versatility

It is VERY important to note that this is a rough outline of desired secondary stats. Your stat weights will be different than others due to varying gear sets and other external factors. The best way to tell what your own stat weights are is a raidbots.com Top Gear sim with Gems and Enchants taken into account. If you don't want to do that then you can just follow this.

Consumables

Enchants and Gems

The openers and rotations listed below are what you want to follow in a normal opener and setting. At times these may change due to fight mechanics or needing to hold CDs. 

 Single Target Opener

1. Precast Haunt

2. Precast Unstable Affliction

3. Agony

4. Corruption or Wither (if Hellcaller)

6. Shadow Bolt/Drain Soul to 2/4 stacks of Shadow Embrace

7. Phantom Singularity

8. Soul Rot  (Trinkets, Potions and Racials)

9. Summon Darkglare

10. Malevolence (if Hellcaller)

11. Malefic Rapture Spam

Be sure to not over cap on Nightfall and Tormented Crescendo procs, which both have a max stack count of 2.

You will hold every Phantom Singularity for every Soul Rot cast, both as Hellcaller and Soul Harvester. 

AOE/Mythic +

1. Seed of Corruption

2. Unstable Affliction (on priority mob)

3. Haunt (on Unstable Affliction mob)

4. Vile Taint

5. Soul Rot (Trinkets, Potions and Racials)

6. Summon Darkglare

7. Malevolence (if Hellcaller)

8. Malefic Rapture Spam

You will want to try and weave Nightfall procs onto your Unstable Affliction target, to fish for Tormented Crescendo procs. Casting Nightfall procs on non-UA targets will not proc TC as of late Beta.

Maintenance Rotation

Afflictions rotation is relatively simple outside of your Malefic Rapture windows. You will want to maintain your DOT effects and Haunt/Shadow's Embrace while refreshing in their Pandemic window. Pandemic is essentially the fact that you want to refresh your DOT's when they reach 30% or less of their remaining duration. This will add that remaining duration onto the new DOT application without any damage being "lost".

While it is not a massive DPS loss to refresh before your Pandemic window, it is still something to min/max. Refreshing DOTs on the move however is a DPS gain (over casting nothing) and is one of the main perks of Affliction, it's incredibly mobile.

DOT Pandemic Range

Agony - 4.5 Seconds

Corruption - 4.2 Seconds

Unstable Affliction - 6.3 Seconds

Outside of DOT maintenance you will have your Malefic Rapture windows every minute which pair Soul Rot with Phantom Singularity or Vile Taint. Malefic Raptures damage is based off Mastery and the number of active DOTs on each mob.  

If you are playing Soul Harvester, you can cast Malefic Raptures more often outside of your Rapture window, due to the fact that Soul Rot grants you 3 Shards when cast. Try not to over cap (outside of the opener) if possible.