Normal: Setesh ignores threat and Chaos Bolts players at random. When he fires Chaos Blast, move away from the target location and avoid the void zone that the Blast creates there. If he casts Reign of Chaos, avoid the little void bubbles that spawn. DPS should try to get a Seed of Chaos buff on them. The tank should gather all the adds summoned from the portals that Setesh creates (two Wurms, a Seeker, and a Sentinel spawn from each portal before it closes) while DPS burn down the boss. Depending on skill/gearing, one DPS might want to help burn down some of the adds to keep the tank from getting overwhelmed.
Heroic: Players must kill the portals or they will continually spawn enemies.
[Chaos Bolt]—Sends a bolt of chaos at the enemy, dealing 10179 Chaos damage. Chaos Bolt cannot be resisted, and pierces though all absorption effects.
[Seed of Chaos]—Deals 3499 Shadow damage to all enemies within 15 yards and plants a seed of chaos, causing harmful abilities to deal an additional 15000 damage.
[Anti-Magic Prison]—Traps an enemy target in an Anti-Magic Prison, causing them be immune to magic and healing effects. Channeled
[Charged Fists]—Increases the casts' attack speed by 30% and Physical damage dealt for 15 sec.
Setesh opens a portal to the void every 20 sec. Each portal summons, in this order, two void Wurm, followed by a Void Seeker about 10 seconds later, followed by a Void Sentinel about 10 additional seconds later.
The tank should tank Void Sentinels. Setesh is not tank-able.
DPS should kill Void Seekers to prevent it from using Anti-Magic Prison. They should attack Setesh otherwise, and should ignore the Void Sentinels.
The tank should be able to kill the Void Wurm by themselves.
Should a Void Seeker cast Anti-Magic Prison, they should be interrupted, stunned, etc as soon as possible.
Portals now summon Sentinels, Worms, and Seekers, in that order.
However, portals can be DPS'd (200k HP) just like the Lord Jaraxxus fight in heroic Trial of the Crusade. DPS should switch immediately.
Portals will explode for shadow damage on destruction, so it might be preferable for the melee DPS to get away from it before it's destroyed.
Total of six portals appear, by this time the tank will be holding six Sentinels and it can get quite tricky. If the group fails to destroy the portals quickly enough there will be extra adds spawned that must be kited or tanked. A moderately geared tank can handle more of the sentinels, as they do not hit hard enough to tax a tank until a large group is spawned. Sentinels can be Rooted or slowed while the tank kites them around. A DK tank can keep up Chains of Ice pretty much constantly on the entire group while running around the boss.
Heroism/Bloodlust/TimeWarp can and should be used from the beginning, during the pull, including Army of the Dead, while all players are free to nuke the boss. Army will not aggro the boss, as he has no aggro table.
You fear that which you cannot control. But can you control your fear?
Setesh's dying quote "Harness your hatred" is taken from Emperor Palpatine in Star Wars: Episode VI - Return of the Jedi
The Egyptian god Set is commonly depicted with a still unknown animal's head. Setesh's model depicts a Jackal's head, which is more like the Egyptian god Anubis, which means Setesh could be a combination of the two and their connection to the underworld, hence the Construct of Destruction.
Setesh may be based on the evil Set, the Egyptian God of storms, chaos and the desert.