GHOULS
One of the most exciting aspects of your Death Knight is ghouls. Here is a handy ghoul guide to get you started.
At level 56 your Death Knight will learn the Raise Dead spell. This spell will allow you to summon your own ghoul. The spell does require either a nearby humanoid corpse to work, or Corpse Dust that you can purchase from a reagent vendor - so make sure you have a ready supply on hand at all times in case there are no corpses handy.
The ghoul that you summon will remain active for 2mins (if it doesn't die sooner from damage) and acts like other summoned guardians, rather than pets. This means that they are unlike warlock and hunter pets in that they are not always able to be summoned and they are not under your direct control.
You are able however, to make certain investments to improve your ghoul with your talents (all ghoul related talents are in the Unholy tree). By spending your talent points on Ravenous Dead, you are able to incrase your ghoul's survivability and damage output. Master of Ghouls will transform your ghoul from a guardian in to a real pet. It is only with this spell that you will gain a pet spell bar and full control over it and its abilities. Also, it will remain indefinately active as long as it does not die. The only draw back here is if your ghoul does indeed die and the cool down on Raise Dead is not yet up. The last option to spend talent points on your ghouls is Night of the Dead. The mechanics of this talent is rather strange, but the effect is that your normal spell rotation will reduce the cooldown on Raise Dead so that it no longer has a cooldown. It is important to remember that you will not be able to ever have two ghouls active at one time and you will not be able to control them without Master of Ghouls, but you will be able to have a ghoul permenantly at your side. It is probably a waste to get both Night of the Dead and Master of Ghouls unless your ghoul is constantly perishing within the 5 min resummon cool down.
Currently there is only one glyph that you can obtain to improve your ghoul. Glyph of the Ghoul will further increase your ghoul's damage. This glyph is only useful if you are going to encorporate your ghoul in to your play style and invest talent points in your ghoul.
Your ghoul's abilties, which come under your control through talents, have a lot of utility built in to them. Controlling your ghoul mean an extra spell bar of abilties to manage if you want to take full advantage of it. The most interesting ablitiy is the 3sec stun with only a 30sec cooldown that should prove to be valuable in arena:
