Shadowburn and Rain of Fire. There are SO many adds in this fight that you'll be responsible for executing them quickly. Your raid will set up DPS priorities on adds, but your job will be to keep Rain of Fire up so that you can Shadowburn infinitely (more than the 4 times alloted). Rain of Fire will generate Embers so fast you won't know what to do with them other than mega-execute adds. Use your Havoc macro to cleave damage onto Horridon. You will almost never target Horridon as there will almost always be an add up which is a priority target over Horridon. Mannoroth's Fury shouldn't be necessary as the adds should be clumped up on a tank. KJC is very useful for moving and casting at the same time to avoid green poison and sand traps.
If you care about min/maxing, I recommend going full Mastery and Crit on this fight. A majority of your damage should be from Shadowburn and Haste affects the casting of this ability about none. You'll want mega-nukes on adds, so be prepared to reforge a bit. Your choice between Sacrifice and Supremacy is yours. I chose to leave my Imp on Horridon and have him switch when needed (Venom Priests/War Bears/Ice Lords).