Cataclysm will ship with three raid instances ready to be played. There are a total of 13 new bosses including the secret heroic-only Sinestra encounter, so players should have plenty to do upon reaching level 85.

Throne of the Four Winds

Location:

The Throne of the Four Winds is part of the Skywall complex which is located high above Uldum. This airborne instance is located in the southwest area of the zone.

Yes. The instance is floating. Yes. The game does look this good.

Bosses:

Conclave of Wind - You'll be matched up against Anshal, Rohash, and Nezir the wind lords on your way to fight the Elemental Lord Al'Akir. Each of these djinn have separate powerful abilities that must be avoided. They also must die simultaneously or they will begin to regenerate.

Al'Akir - This is a three phase fight that will pit you against the Elemental Lord of Wind himself. Al'akir is a formidable foe with 12 unique abilities each more deadly than the last.

The Throne of the Four Winds is a two boss instance and as such isn't particularly big.

Bastion of Twilight

Location:

The Bastion of Twilight is located in the Twilight Highlands in the fortress of the Twilight's Hammer. The Twilight Citadel is a gigantic spire located on the border of Loch Modan and the Twilight Highlands and instance access can be gained through the portal high atop the spire.

Atop the highest spire in Twilight Highlands, the Bastion of Twilight awaits.

Bosses:

Halfus Wyrmbreaker - This large ogre comes with an entourage of flying wyrms (hence the name Wyrmbreaker). The dragonkin that he controls must be defeated to make Halfus vulnerable and killable within his 6 minute enrage. He is the first boss in the Bastion of Twilight.

Valiona & Theralion - This boss consists of two large Twilight Dragons. They share the same hit point pool and have different sets of abilities. Dealing with their abilities and cleanly executing the fight are key to defeating this boss.

Ascendant Council - Some of the elite of the Twilight's Hammer cult have ascended to an elemental state. Here you will face four of them at once: Earth, Wind, Fire, and Water. When you've defeated their individual elemental forms they will merge into the Elementium Monstrosity which possess the power of all four elements.

Cho'gall - At last you will face the leader of the Twilight's Hammer. Cho'gall, the ancient ogre warlock, has a wide variety of dangerous abilities and as a servant of the Old Gods he has access to a powerful form of corruption that will transform you into a faceless one and become immune to healing. If your raid is strong enough to defeat Cho'gall on heroic difficulty then you are in for a surprise…

Sinestra - Upon defeating Cho'gall on heroic difficulty the floor will collapse and you will find yourself in a cavern beneath the Twilight Citadel where you will find Sinestra, Deathwing's consort. Little is known of this fight other than that it is a 'super-heroic' fight similar to the Algalon the Observer encounter in Ulduar.

The Bastion of Twilight is the largest of the three new raids due mostly to the separate area for Sinestra.

Blackwing Descent

Location:

Blackwing Descent is located at Blackrock Mountain which you can access through the Burning Steppes or Searing Gorge. Players that have completed Blackwing Lair will find the entrance to the instance nostalgic: it is accessed high on the Western side of Blackrock Mountain through the room where Vanilla WoW raiders first fought Nefarian.

Bosses:

Magmaw - This boss is like Ouro the sand worm if you dipped him in magma and made him spit fire. This is one of the first two bosses along with the Omnitron Defense System. You can choose which one you want to do first.

Omnitron Defense System - In this encounter you'll fight four Golems that act as the guardians of the Blackwing Descent. The fight begins with one Golem active and as the fight progresses three additional Golems will activate. Players must kill all four to proceed.

Maloriak - This boss is a dragon-humanoid hybrid. He throws vials into a cauldron which trigger special highly dangerous abilities. When pushed to 20% health he gains additional abilities and attacks relentlessly; raiders will need coordination and great execution to prevail.

Atramedes - A large flying dragon, this encounter has one of the most unusual mechanics in recent memory. Atramedes is blind and relies on sound to hunt, players will gain a 'sound bar' when they attempt this encounter and if any player's bar reaches 100 Atramedes will annihilate them quickly regardless of their gear or whether they are a tank. Players must use the gongs placed around the room to disorient and stun Atramedes and to reset their sound bar.

Chimaeron - This boss is a massive three headed Hydra. He has some bizarre abilities: as you fight him there is a robot running around the room spraying slime. This slime will make you immune to death if you have more than 10,000hp. You'll need to make smart use of this slime so your raid isn't one shot by Chimaeron's powerful AoE attacks.

Nefarian - He's back! Both Onyxia and Nefarian are back from the dead and ready to play. You'll begin the fight with a rematch against Onyxia. You'll need to quickly dispatch her before her damaging aura grows out of control. After 30 seconds Nefarian will land and must be separated from Onyxia as they have a buff they gain when in proximity to each other. Reanimated Bone Warriors will soon join the melee and you'll be dodging shadowflames, lava, and Shadowblaze which is a type of 'smart fire' reminiscent of Mimiron and Archimonde's. You'll have your hands full with this long and complicated fight.

Blackwing Descent offers a lot of flexibility in terms of the order in which you fight its bosses.