In lore, it was noted for its famous occupant, the last Guardian of Tirisfal, Medivh. After Medivh had been defeated by the army of Azeroth during the First War, Karazhan was abandoned and has remained in that condition. The arcane powers, however, have never abandoned the tower.

The Ivory Tower of Karazhan, once the dark wizard Medivh's seat of power, stands in the center of this jagged region and leaks evil magic into the land. The trees are petrified and leafless, and a terrible wind keens through the canyon. Ogres, demons, undead and a few human gypsies roam the interior. Deadwind Pass connects Duskwood and the Swamp of Sorrows, and travelers are advised to move quickly - whispers tell that a new force has claimed the Ivory Tower, and that the ogres and demons are under its sway.

Information adapted from: WoWWiki - Karazhan

Attumen the Huntsman: Attumen the Huntsman is usually the first encounter in Karazhan, since he is also the easiest.

Moroes: Moroes is the second encounter in Karazhan and he is not optional, as he unlocks the opera event which leads to The Curator.

The Maiden of Virtue: The Maiden of Virtue is an optional boss in Karazhan.

Opera : The Big Bad Wolf: One of three random opera events - influenced by the Little Red Riding Hood stories.

Opera : The Wizard of Oz: One of three random opera events in which the player fights a total of 5 mobs based on The Wizard of Oz.

Opera : Romulo & Julianne: One of three random opera events - based on Shakespeare's Romeo and Juliet.

The Curator: The Curator is often considered to be the first true gear check in Karazhan. This fight is required in order to reach the rest of the encounters in the instance.

Terestrian Illhoof: Terestian Illhoof is an optional boss in Karazhan that can be found behind a "conspicuous bookcase".

Shade of Aran: Shade of Aran is an undead boss located in the library of Karazhan.

Netherspite: Netherspite is an optional boss in Karazhan.

Chess: The Chess Event is a required encounter in Karazhan in which the players control pieces on a giant chess board.

Prince Malchezzar: Prince Malchezaar is a boss located at the top of Karazhan.

Nightbane: Nightbane is a summonable skeletal dragon in Karazhan, considered by many to be the final encounter of the instance.