Gehennas

Gehennas is a Shadow-based Flamewaker with two Flamewaker Elite guards in Molten Core

Gehennas:
Deals 2000-2500 Shadow damage to a non-tank target. AoE that deals 925 to 1075 Fire damage every 2 sec. for 6 sec. Radius: 10 yards Reduces the effect of healing abilities on nearby enemies by 75%. Type: Curse Radius: 45 yards
 * Shadow Bolt
 * Rain of Fire
 * Gehennas' Curse

Flamewalkers
Stuns nearby players for 4 seconds. Radius: 8 yards. Reduces the target's armour by 1000 per stack. Stacks up to 20 times. Deals normal melee damage plus 35.
 * Fist of Ragnaros
 * Sunder Armour:
 * Strike

Strategy
Clear the entire area around Gehennas and then pull the guards to the south west and tank Gehennas in the center of the room. Make sure Gehennas is far enough away that the only people in range of his curse, shadow bolt, and rain of fire are the main tank and his healers/decursers. The two guards need to be tank far away from Gehennas and need to be kept a decent distance apart, otherwise the tanks and melee DPS will end up stun locked the entire battle from the two cleaves. The MT on Gehennas will need a dedicated decurser usually a mage, but a druid will work as well. The tank must have the curse removed as soon as possible. Gehennas deals a fair amount of damage and you will not be able to keep the tank up if he is only receiving 25% of the healing. Once the two guards are dead the entire raid should focus on killing Gehennas. The guards go down pretty quickly, Gehennas takes a while.

Mages and druids in this fight should be sure to adjust their decursing priority list to place the MT at the top, followed by other melee players, as the curse greatly reduces the amount of healing a player can receive.