After defeating Lord Jaraxxus Grom Hellscream gets all up in arms thinking that the Alliance is purposely trying to summon demon lords (Wilfred is a Gnome, after all). Varian gets all mad because, as logical as Grom's point is, he's a jerk and will use any excuse to challenge the Horde. The end result is that, even though the third boss fight was supposed to be against various Argent Crusade champions, you end up fighting the opposing faction's champion's instead.
On 10 player, your raid will fight 6 disproportionately strong faction champions, and on 25 player, 10. These 6 or 10 are randomly selected from a list of 14, but there will always be 2 healers in 10-man and 3 healers in 25.
On normal difficulty every champion has a PvP trinket that they will use right away. A good way to negate it early on is to have a Warrior Charge in and use Intimidating Shout. On heroic modes they each have two PvP trinkets. Plan accordingly.
This fight is considered to be "PvP", and as such everything is subject to Diminishing Returns, and crowd controls have PvP durations. For example, if you cast Fear on a target, they will be feared for 8 seconds. If you cast it again, it will last for 4. Again and it will last for 2. Again and they will be Immune until the DR timer resets.
Frost DPS Death Knight - Uses Chains of Ice, Strangulate, Frost Strike, Icy Touch, Icebound Fortitude, and Death Grip. He will hit pretty hard on soft targets. Middle priority kill target.
Balance Druid - Uses Barkskin, Cyclone, Roots, Faerie Fire, Force of Nature, Insect Swarm, Moonfire, Starfire, and Wrath. All in all actually a more helpful than harmful opponent. It will use Cyclone quite often which can save players' life as it removes DoTs and makes them damage immune. Low priority kill target.
Restoration Druid - Uses Barksin, Nourish, Lifebloom, Rejuvenation, Regrowth, Tranquility, Nature's Grasp, and Thorns. Primarily heals through his HoTs, which tick for massive amounts (ex. Rejuvenation ticks for 50k alone). Can be ignored so long as you have a reliable Shaman, Mage, or Priest Purging/Spell Stealing/Dispelling (preferably all three). He can also be locked down by a Warlock alternating Fear and Banish. High or low priority kill target, depending on strategy.
Survival Hunter - Has a Cat pet, uses Aimed Shot (reduces healing done), Deterrence, Disengage, Explosive Shot, Frost Trap, Steady Shot, Wing Clip, and Wyvern Sting. Low damage, can usually be ignored. Lowest priority kill target.
Arcane Mage - Uses Arcane Barrage/Explosion/Blast, Blink, Counterspell, Frost Nova, Frostbolt, and Polymorph. High damage with lots of counterspelling and crowd control. High priority kill target.
Holy Paladin - Uses Cleanse, Divine Shield, Flash of Light, Hammer of Justice, Hand of Freedom, Hand of Protection, Holy Light, and Holy Shock. Lowest priority healer kill, easy to interrupt. Will cleanse allies so kill early if using much crowd control. Middle priority kill target.
Retribution Paladin - Uses Avenging Wrath, Crusader Strike, Divine Storm, Hammer of Justice, Hand of Protection, Judgement of Command, Repentance, and Seal of Command. Moderate physical damage, but very helpful to his allies. Middle priority kill target.
Discipline Priest - Uses Dispel, Flash Heal, Mana Burn, Penance, Shield, Psychic Scream, and Renew. Primarily casts shield, renew, and fear. Doesn't really need dedicated interrupts as he doesn't use spells with cast times often. Middle-to-low priority kill target.
Shadow Priest - Uses Dispel, Dispersion, Mind Blast/Flay, Psychic Horror, Psychic Scream, Shadow Word: Pain, Silence, and Vampiric Touch. Low direct damage, mostly DoT-based. Annoying with fear and silence effects. Middle-to-low priority kill target with Priest and Paladin healers in raid, middle-to-high otherwise.
Subtlety Rogue - Uses Blade Flurry, Blind, Cloak of Shadows, Eviscerate, Fan of Knives, Hemorrhage, Shadowstep. Very high damage, especially on cloth. High priority kill target unless you have a Death Knight use Chains of Ice / Death Grip / Dark Command to keep it locked down and out of the raid.
Enhancement Shaman - Uses Earth Shock, Heroism (Bloodlust), Lava Lash, and Stormstrike. Extremely high melee DPS, difficult to lock down. Usually #1 priority kill target. Heroism/Blood lust must be Purged/Mass Dispelled ASAP.
Restoration Shaman - Uses Cleanse Spirit, Earth Shield, Earth Shock, Heroism, Hex, Lesser Healing Wave, and Riptide. High priority kill target - most effective healer. Difficult to lock down. Heroism/Blood lust must be Purged/Mass Dispelled ASAP.
Affliction Warlock - Felhunter pet, Uses Corruption, Curse of Agony, Curse of Exhaustion, Fear, Hellfire, Searing Pain, Shadowbolt, and Unstable Affliction. Mainly DoT based with light direct damage. Will use Hellfire in the middle of the group which deals a ton of damage. Low priority kill target with Priest/Paladin healers in raid and moving out of Hellfire.
Arms Warrior - Uses Bladestorm, Charge, Disarm, Intimidating Shout, Mortal Strike (reduces healing done), Overpower, Retaliation, Shattering Throw, and Sunder Armor. Very high damage, high priority kill target.