The Violet Hold is laid out like Black Morass, but much smaller. The Violet Hold is the high security Dalaran Prison that the Blue Dragonflight has infiltrated, and is releasing prisoners to sabotage the city. Your party has charged in to save the day and give the prison guards some respite. They get out of the way, seal the main gate behind you, and it's your job to be sure that gate holds up. So over time, prison cells are opened up, and the mobs within come your way in waves, just like Black Morass. Except you can replace Medivh's whining with the sweet, sweet silence of a door.
The bosses themselves really are just a nod to the game of yesterday. You have bosses borrowing abilities both from WoW Classic (Viscidus, Jin'do, Magmadar, Azuregos) as well as some abilities and baddies of The Burning Crusade (various Arakkoa and Ethereal bosses, among others). You get two random bosses each time, and then Cyanigosa, a Blue Dragon, is the last boss.
When this dungeon was first implemented in the Wrath beta, there were a lot of complaints that it was Black Morass 2.0. When I first went inside I kind of agreed with that, it almost is the same dungeon. After actually running the dungeon couple of times I realized what it really is. It's a nod to us. A nod to the people that have been around forever. It's saying, "Hey, remember that first week in Molten Core? Remember the first time you fought Magmadar? Yeah, welcome back. Now let's see how you handle it this time." And after you've completed it, you get to fight Heroic Magmadar. Hell yeah!