The Bastion of Twilight is a raid whose entrance is located in Twilight Highlands in the World of Warcraft game. The minimum level for this dungeon is 85. The instance was released in the Cataclysm expansion. The raid contains 5 bosses. The end boss of The Bastion of Twilight is Sinestra.
He'll have his back to you but once you start, he'll come after you with all his might. His main power is shadow and when he casts, you'll not be able to receive any healing. As you go round, you will be able to unlock the dragons if you so wish.
Valiona is actually two bosses here, the other one being Theralion, both being dragons. When one dragon is on the floor, the other will be flying above. The bosses have shared health so you only need to target one of the bosses to get them down. The one from above will fire a large amount of AOE damage whilst the one on the ground will just do melee damage against whoever is in front of them. Be sure to clear the trash from the previous room otherwise they will follow through into the room with the dragons.
There are three phases in this, the first two phases are essentially the same where you take on a pair of Elemental adds. When you've dpsed them down sufficient amount, the next pair comes. When the second pair have been dpsed down a bit, the four elementals merge into the Monstrosity. With the Monostrosity, there is a large amount of AOE damage to be aware of and need to kite the boss round to avoid the crap on the floor.
Shoot him to start the fight. You need to kite this boss round and make sure that you don't step in any of the AOE damage. He'll create adds in the room which you need to kill before they kill you. There will be a lot of aoe damage, both stuff that he fires and stuff that emanates out from where he is standing so you need to move out of the way. If you make sure you don't get too much corrupted blood, you can receive an achievement for it.
At the end of the battle on heroic, you'll fall through the floor which will collapse. The bosses body and loot will fall as well. Only heroic will allow you to fight the final boss, a giant female drake.
Sinestra is only available on the Heroic raid level. After you've killed Cho'gall, the floor will open and you will fall down along with the loot. If you die here, you resurrect at the start of the raid and its a long walk back. Its a three phrase battle.
The first phase is simply to take Sinestra out. When Sinestra looks like she's dead, she'll come back alive again and Calen will arrive to help you. You need to destroy the pulsating eggs, one each side of Sinestra before she is able to overpower Calen with her ray.
The third phase is to simply kill Sinestra. In each of the phases will be adds that will target you and she'll create AOE damage on the floor to be avoided.