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.

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.