2-Piece

Diabolist

Chaos Bolt and  Rain of Fire summons a Demonic Oculus, up to 3. Consuming Demonic Art flings your Demonic Oculi at your current target, exploding for (457.103% of Spell power) Fire damage and (304.702% of Spell power) Fire damage to enemies within 10 yds.

Hellcaller

Mark of Xavius increases  Blackened Soul damage by an additional 3% per stack.  Malevolence increases the stack count of  Wither by an additional 2 stacks

4-Piece

Diabolist

Demonic Oculi analyze the battle while active and deliver information to you as they explode, increasing your Intellect by 2% for 10 sec.

Hellcaller

Casting  Malevolence grants 2 stacks of  Backdraft and prevents  Blackened Soul from consuming stacks while dealing damage 15% faster for 8 sec.

Hellcaller Destruction Single Target

Hellcaller Destruction Havoc Cleave

Hellcaller Destruction M+ 

Diabolist Destruction Single Target

Diabolist Destruction Havoc Cleave

Diabolist Destruction Mythic + AOE 

Intellect>Haste>Critical Strike>Mastery>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 Soul Fire

2. Summon Infernal (Trinkets, Potions and Racials)

2a. Malevolence (if Hellcaller)

3. Conflagrate

4. Chaos Bolt

5. Channel Demonfire (If Talented)

6. Conflagrate

7. Chaos Bolt

8. Incinerate

9. Resume Maintenance Rotation

Shadowburn usage has changed in Patch 11.2. If talented, you will want to only use the spell on the move OR if a mob is going to die and refund the shard spent. 

If you are playing Hellcaller, Malevolence grants 2 stacks of Backdraft on cast with 11.2 Tier Set. Backdraft's max stack count is 2, so be sure to not over cap.

If you are playing Channel Demonfire, cast it on cooldown. 

AOE/Mythic +

1. Cataclysm (if talented, to mass apply Wither as Hellcaller)

2. Summon Infernal (Trinkets, Potions and Racials)

3. Malevolence (if Hellcaller)

4. Rain of Fire

5. Channel Demonfire

6. Conflagrate

7. Rain of Fire Spam (3+ Targets, unless there is a priority mob).

If you are playing Channel Demonfire, be sure to cast it on CD in AOE, to extend the duration of your Wither/Immolate effects. 

Mayhem is your default cleave talent choice over Havoc in Mythic +. Be sure to keep an eye on where the effect procs, if target swapping is needed. 

Havoc Usage

There are a couple different ways to use your Havoc window which depends on the setting you are in. At times it will be used for shard generation to fuel more Rain of Fire casts. If you are looking to use Havoc in a setting where there are two targets, there are a few general guidelines you can follow. 

1. Try to enter the Havoc window with 2.5-3.5 Soul Shards.

2. Apply Immolate/Wither to both targets as long as they are going to live for 6+ seconds.

3. Be sure to not shard cap and be mindful of shard generation with Conflagrate/Incinerate casts.

4. Soul Fire WILL generate 2 shards and fully cleave if cast via Havoc.