For those of you that don't remember Violet Hold from Wrath of the Lich King, the premise of the dungeon is essentially you are stopping prisoners from being released. The prisoners in lore have even included characters such as Prince Kael'thas of the blood elves. When Kael'thas broke out of the Violet Hold, the Kirin Tor instead took over command of guarding the prison.
The dungeon will see players battle waves of enemies just as in Wrath, with a boss being spawned every 6 waves. There is also an additional last boss which will appear after the third boss has been killed.
While it is not overly difficult, the dungeon's trash has a lot of pitfalls for players that are hoping to ignore tactics. Each portal spawned will either create a wave of elite legion monsters or it will spawn a Portal Keeper. The Portal Keepers will continuously spawn mobs until they are killed. One of the mobs found in the dungeon, the Eredar Shadow Mender, will spawn as part of the elite legion group. They heal every ally within 20 yards for a percentage of their shadow damage dealt. If you try to ignore their spells or don't move from their large AoE, you might find yourself unable to kill the group!
As far as bosses go, there are a number of possibilities. I personally have only experienced some of them, since RNG is always a factor despite running the dungeon multiple times.
In regards to mechanics, it looks like quite a few of the fights are extremely simple, but we will have to see if they add anything else.