And it's almost over. The fourth and fifth boss encounters are much more epic than the previous encounters, actually reaching the difficulty of some Ulduar bosses.
Boss Encounter 4: The Twin Val'kyr
Before the bosses enter, designate who will take the Dark Essence or Light Essence buff.
When you start the encounter, the Dark Val'kyr will move south and the Light Val'kyr will move north. They will stop close to the northwestern and southwestern points of the star design on the Coliseum's floor. The essences will be on the northwestern, southwestern, southeastern, and southwestern.
You will need two tanks for these bosses. If you're relying on a Paladin tank, have them tank the Dark Valkyr. Paladin tank skills will have very little effect on the Light Valkyr.
The tanks should pick the essence that is the same color as their target boss to reduce the amount of damage that they take. Classes with threat-directing skills will need to use those skills for the duration of the encounter.
This fight is healing-intensive, comparable to Ignis the Furnace Master in Ulduar. Surge of Light and Surge of Darkness will deal raid-wide damage every two seconds from the very start of the encounter. You can minimize the damage from one of the surges by picking an essence. The damage from the opposing element will increase, but the overall damage is lesser than not picking an essence at all.
As you go through the fight, light and dark spheres will appear at the edges of the arena. This happens before one of the Valkyr Twins casts a spell. Everyone in the raid should collect spheres that match their essence. They will take a little damage, but nothing compared to taking a sphere of the opposing element. If the bosses collect these spheres, it will add to their empowerment and once they've reached a certain number, will soft-enrage and deal considerable damage to the tanks for 15 seconds.
A popular tactic is to assign all ranged DPS to one essence and melee DPS with one healer to the other essence.
Ranged DPS should clump to the point that your character models overlap. During the sphere phase, the healer assigned to the melee DPS should filter any of the spheres that don't match the ranged DPS' essence. Melee DPS should do the same but to protect the tank in charge of their target boss. Ranged DPS will take in any matching spheres that are within 5-10 yards of their assigned position.
Once a player reaches 100 stacks of "Powering Up", they will gain Empowering Light or Empowering Darkness, increasing damage dealt. Any threat-direction skills that they have should be used on the tanks before the massive surge of damage pulls the boss from the tank.
Post-Spheres: Boss Skills
After the Sphere Phase, one of the bosses will cast either a Vortex or Twins' Pact. All players except the tanks should change their essence to match the boss casting the Vortex. Overheal the tank that doesn't have a matching essence. Twins' Pact is a 15-second cast that heals 20% of their total HP and cannot be interrupted until the boss casting it has their shield broken. Breaking the shield DOES NOT stop the cast, so give anyone who can interrupt the time to do so. If your DPS is good, you won't need to change your essence to take down the shield while using the setup mentioned in Sphere Collection.
There is no set pattern for the spells, but the spells cast by the bosses will not repeat until the two Vortexes and two Twins' Pacts have been cast.
Boss Encounter 5: Anub'arak
You can forget about the celebration for beating the first four bosses of the Trials. Just as everyone is about to get their mug of ale, the Lich King crashes the party and nerd rages at Tirion for not inviting him. As payback, he shatters the floor and sends you all plummeting into a pool of water while claiming the rights to your souls. He'd have done a better job if he hadn't forgotten about WoW's water physics.
At least the scenery beats the monotony of the Argent Coliseum. The frozen kingdom of Azjol-Nerub towers before you and at the far end of the frozen walls is Anub'arak, back to redeem himself after being such a mediocre five-man boss.
You'll need two tanks, one for Anub'arak and one for the Nerubian Burrowers.
Assign one ranged DPS or one Death Knight to destroy the Frost Spheres floating above the raid for the duration of the encounter.
Phase 1: Tank-and-Spank
Your designated Sphere Breaker should take down at least 3 Frost Spheres during Phase One. These will create patches of Permafrost which will stop the Nerubian Burrowers from burrowing before the raid can kill them.
The main tank should tank Anub'arak near one of these patches. The off-tank should take the Nerubian Burrowers to the same patch used for Anub'arak.10-man encounters only see one Nerubian Burrower spawn every few seconds, so DPS should focus fire on every second Nerubian Burrower that spawns and then switch to area-of-effect skills to damage the Burrower and Anub'arak.
Anub'arak is a pushover to tank, dealing basic damage and stunning the tank every now and then. He also casts a damage-over-time spell on a few raiders, but it's nothing that can't be outhealed.
Nerubian Burrowers attack at a faster rate when they are near each other. One off-tank is often enough if the DPS can quickly kill those Burrowers.
If you have Deadly Boss Mods, the assigned Sphere Breaker should get to work 20 seconds before Anub'arak burrows.
And Anub'arak will burrow. No puny patch of ice will stop him.
Phase 2: Run, Period.
Once underground, the raid needs to get as far away from Anub'arak while keeping a patch of ice between them and him.
Anub'arak once again uses Impale, but he will chase a specified target this time. Players targeted by Anub'arak will have a mark similar to Hunter's Mark above their heads. Anub'arak' spikes speed up the longer they are uninterrupted by a patch of ice. This phase ends after a certain amount of time, not the number of times Anub'arak will have to crash into the ice patches. If you want to practice for hard mode, kite the spikes around.
Throughout this phase, Swarm Scarabs will spawn around the raid. Everyone should attack these adds even if they're not a tank. Swarm Scarabs don't hurt a lot, but they have a stacking damage-over-time effect that really hurts once you reach 3 or 4 stacks.
Once the timer is up, it's back to Phase 1. Kill the Swarm Scarabs before going back to Anub'arak.
If you don't want him to enrage, Anub'arak should only go into this phase twice.
Phase 3: They'll Crawl All Over You
Once Anub'arak reaches 30%, he'll cast Leeching Swarm. There will be no more Nerubian Burrowers for this phase so you won't need the Frost Spheres.
Anub'arak will heal himself for 10% of the current health of each raid every second for this phase until his death, so DPS will need every skill they can muster. If your DPS can't keep up with Anub'arak, let their health drop to 75-50% to reduce the health drained by Anub'arak.
And with Anub'arak down, the Heroic version of your raid is unlocked.