Here is how I handled the fight. There are a few ways other Warlocks have been able to handle this fight. I'll explain how I handled the fight.
Pre-start - Position your gateway to LoS the boss, as well as you can drop your Demonic Circle in sight like mine is. I highly suggest you mouse-over Kanrethad after he becomes visible. The moment your icon switches from glove cursor to a sword cursor, pop a pre-pot.
He will first start to summon the Pit Lord. Shadowfury the Pit Lord and Cast Enslave Demon. Be aware of the Pet's skills. Using Charge will Interrupt Cataclysm and Chaos Bolt (I used it for Cataclysm) and stun for 6 secs. Remember when you use the Charge to follow up with your own Chaos Bolts! The boss takes increased damage during the stun. The Make sure to use the pet's heal in conjuction with Unbound Will so you will be able to clear DoT's on you and heal yourself up to full.
Before the first set of summoned adds (Imps) he will cast a Chaos Bolt. I used the Gateway method to LoS him. Once the cast is finished, you may Teleport back out via Demonic Circle. He will attempt to cast the 1st Cataclysm, Charge stun him. Following this, he will start to summon Wild Imps. I chose to keep Rain of Fire and a F+B Immolate on the adds, while chain casting Chaos Bolts on the boss. Use the Pit Lord Breath skill to take out most of the Imps.
His 2nd Chaos Bolt will be coming while the Imps are out (unless you brought them down), be ready for it. Following that he will cast Cataclysm again. Charge stun him and immediately following the Charge move the pet as far as you can away from the portal.
Felhunters will start to come out (3 total) in succession. I saved my Potion and my DS/BF for these and Casted a Chaos Bolt on each. Others have been able to Shadowfury one and use a Chaos Bolted Havoc.
You will lose control of your Pit Lord around this time as well, be sure to Re-cast Enslave Demon, as well as another Chaos Bolt. Be sure to Gateway dodge or pop Sacrificial Pact. A Cataclysm will be coming up next, use the Charge stun again.
Next up is the Summoned Doomguard. Keep it Banished from here on out. He will proceed with another Chaos Bolt attempt and Cataclysm. Handle them just as before.
After the Cataclysm he will repeat the phases starting with the Wild Imps. I was closing in around 20% coming up to this part of the fight. Once he summoned the Imps I kept Rain of Fire on the adds along with a F+B Immolate. You will be flooded with Embers, so proceed to chain cast Shadowburns..
Keep dodging Chaos Bolts, and charge stunning Cataclysm.
Grimoire of Sacrifice
TukUI - Frames
TellMeWhen - Procs/Debuff Tracker
Skada - Dmg Meter