Two of the mid-range dungeons in Wrath of the Lich King are the Violet Hold and Drak'Tharon Keep. While there is a fairly long storyline that brings you into the latter, the buildup for Violet Hold is relatively simple. You receive notice from Rhonin, the leader of the Kirin'Tor, that there is a jailbreak in progress at the prison in the center of Dalaran. Malygos has discovered a weakness in the building's roof, and is sending waves of his forces through in an attempt to break through into the city. You and your party are locked into the prison, and must survive multiple waves of enemies and bosses in order to protect Dalaran.
There are 18 waves of enemies altogether, and bosses come in during the 6th, 12th and 18th waves. The first two bosses are randomly drawn from a pool of six, but the final boss is always Cyanigosa, a blue dragon and lieutenant of Malygos. This dungeon has the added element of a door that you need to protect. Not only do you need to ensure that you and the rest of the party survive, but if the door is destroyed, you will fail the dungeon.
Drak'Tharon Keep is located between the two zones of Grizzly Hills and Zul'Drak, and continues the story of Drakuru, which began in Grizzly Hills. As you remember, Drakuru was a captured troll who persuaded you to help him eradicate certain clans of trolls all around the Grizzly Hills. His final task for you was to enter Drak'Tharon Keep, fight your way to the top, and use a special incense at a brazier in the keep's summit.
The Keep has been overrun with Scourge, who are killing the troll inhabitants and raising them up as zombies and ghouls. The bosses range from an undead troll berserker...to a full-blown Lich.
Your path to the summit leads through the raptor pens, where the resident trolls keep their future mounts. A nasty surprise awaits you, however, as the pens also contain a giant devilsaur.
The final boss is a skeletal wind serpent, who will occasionally temporarily rob you of your flesh, which render your regular attacks invalid.
After you defeat the boss, and use the incense at the flame, as ordered, Drakuru appears, not in a vision like before, but in person. You have freed him from his cage in the Grizzly Hills, and as you might have guessed, you have once again inadvertently aided evil. He summons a portal, and who should step through but Arthas.
Drakuru reveals that you have helped him betray his people to serve the Scourge, and as a reward, Arthas grants him a new, powerful undead form. For your blind assistance, you are spared by Arthas once again, and Drakuru offers you a lofty position if you seek him out in Zul'Drak.