Boss Characteristics
The Illidari Council is comprised of four blood-elf bosses who share the same health pool, meaning that any damage done to one of them is done to all of them. Each boss has abilities similar to player classes: a mage, a priest, a paladin and a rogue. The four of them work together to survive. For example, the priest (Lady Malande) heals the other bosses during the fight. It's important to deal with each boss according to his abilities.
Raid Composition
The Black Temple is a 25-man raid and the recommended composition for the Illidari Council should be one main tank for Gathios the Shatterer, two off-tanks for Lady Malande and Veras Darkshadow, a mage for tanking High Nethermancer Zerevor, 12 damage-dealers, and 9 healers. If you prefer, you can exchange one healer for a damage dealer. Each player should be prepared for using whatever abilities they have to interrupt the bosses' spells, particularly Lady Malande's healing spells.
The Pull
With four bosses to handle at once, it's critical to "pull," or attack them, in such a way that each boss goes to the appropriate tank in a strategic location. A mage should first pull High Nethermancer Zerevor away toward the left of the platform using a ranged spell. The main tank should pull Gathios the Shatterer either through direct engagement or through misdirection and fight him to the right of the platform. The off-tanks can pull the other two bosses and fight them near the middle of the stairs, separated from each other by a safe distance.
Winning the Fight
Along with the off-tank, three other damage-dealers who can interrupt spells should be attacking the healer, Lady Malande. All four should be taking turns interrupting as many of her spells as possible. All mages should be using their Spellsteal abilities to stop High Nethermancer Zerevor's Dampen Magic spell as often as possible and all players should stay at least 10 yards away from him. All other damage-dealers should be focused on attacking Gathios the Shatterer until he is dead. Remember that killing any one boss will effectively kill them all.