For Heroic Horridon, there is really only ONE major difference between Normal and Heroic.  For a review of normal mechanics, go here...

Adds have much more health and deal significant more damage.  Need very strong AoE/Burst dps to kill them quickly before we get overwhelmed.  In addition to their increased damage and health, Horridon will use his new ability which will buff them further.

The new mechanic is as follows

1. Dire Call - emits a powerful roar, dealing 250k physical damage to the raid, increases the melee attack speed of the adds by 50%.  Also summons a Direhorn Spirit.
   1. Direhorn Spirit - Summons a direhorn spirit (red in color) and fixates on a random player.  ONLY THE FIXATED PLAYER CAN SEE THIS DIREHORN.  They have over 420 mil health so killing them is simply not possible since only 1 player can see them. If you are hit by this spirit you will be 1-shot.
      1. Weak Link - the spirit link from the Direhorn to the player is "weak", any attack made against it will knock it back.  So essentially the way to counter this Spirit is to ranged hit it and knock it back a ways so you have time to dps the adds/boss.  Hunters and warlocks are great for this as they can just send their pets on the spirit for a while and knock it back a fair distance.
         1. DO NOT knock it back through a wall as it has been knocked back through walls and reappeared immediately in front of its target and insta-killed them. 
         2. DO NOT use invisible, feign death, etc or anything that will cause you to completely drop aggro.  This can cause the spirit to drop its fixate on the player, and will cause it to focus on a different player who cannot see/target it to knock it back.