The Fifth Boss
Strategy
The Twin Emperors share a percentage life total, so when one loses life, the other loses the same percentage of their max HP. Emperor Vek'nilash deals physical damage and can only be damaged by physical sources, whilst Emperor Vek'lor is a caster who can only be damaged by magic. Therefore, you'll have to tank Vek'nilash with a Warrior or Druid, while a Warlock with good Shadow resistance gear should tank Vek'lor.
You should separate the two Emperors from each other to prevent them from healing one another with Heal Brother (30,000 HP per second). They will also use Twin Teleport every 30-40 seconds, swapping places with their twin during the fight. Your tanks should be poised to prevent each boss from reaching its twin and healing them. Your DPS should also swap after each teleport to keep their damage output optimal.
Vek'lor, the caster, will use a plethora of different abilities, such as: Blizzard, which deals damage and slows the movement speed of all players caught in the AoE, Arcane Burst, which knocks back nearby players, and Shadow Bolt, which damages the primary target.
Vek'nilash, on the other hand, deals physical damage: Unbalancing Strike deals heavy damage to the target and reduces their defense skill, while Uppercut knocks back the tank drastically. Don't forget to have players use Disarm one after another, as well as constantly use Demoralizing Shout and Thunderclap on the boss.
In addition, neutral insects already in the room will turn hostile and try to wipe your raid. A group consisting of 3 Mages, 2 Warriors, and a healer should focus on taking down these creatures throughout the entire fight.
After 15 minutes of combat, the two Emperors will enrage and one-shot your players. You'll need to optimise your damage output and heals to kill them before this point.