No matter when you want to fight against a Death Knight, there is more than just the plate wearing and glowy eyed psycho in front of you to worry about, but what is more there is the pet to deal with. Depending on talent expenditure, the threat this little beast poses to you differs greatly but is always something you need to pay more attention.
   There are two types like pets and minions you need to concern when it comes to Ghoul. Each Death Knight can summon a Ghoul minion for two minutes which is not under the player control. In general, this minion is weak with low damage output and health while being highly susceptible to Area of Effect effects.
   With a minion Ghoul, the main problem is that it is an excellent target for the Death Knights ability Death Pact; this sacrifices the ghoul in order to restore 40% of the players' health. This sacrificial heal can crit for a 60% heal and can be further boosted by Vampiric blood; when fighting against Death Knights it is in your best interests to prevent this.
So how do you prevent the Death Knight from sacrificing his Ghoul? Those who plan on sacrificing their minion only summon it when they need it and while the Ghoul is clawing its way out of the ground it is unable to be targeted by Death Pact preventing an instant sacrifice. As soon as you spot the Ghoul being summoned you should use whatever incapacitate or silence effects you have on the Death Knight before quickly killing the Ghoul.
If the Death Knight should be foolish enough to summon the Ghoul before he requires it, make sure that you can kill it first. He won't have the runic power to instantly sacrifice it but there will be little to no damage on him to be healed.
Speaking of dealing with pet Ghouls, things become a little more difficult; unholy Death Knights have the ability to precisely control their Ghouls, giving them specific targets to attack and being able to recall the ghoul safely from danger. When the Ghoul is controllable it gains the ability to stun its target for 3 seconds every 30 seconds in a similar manner to a Hunters Intimidate skill.
   Not only have pet Ghouls large amount of health but they can huddle to reduce all damage taken by 50% so simply Crowd Controlling the Death Knight while killing the Ghoul is rarely a good idea as it requires significant effort. Furthermore, this unholy Death Knight can resummon his Ghoul far sooner than any other spec due to the Night of the Dead talent.
   Threat becomes apparent due to this speedy re-summon and the pet Ghouls' higher health pool. A Death Knight is capable of commanding his Ghoul to explode by casting the spell Corpse Explosion on it where it will detonate after a short time for 25% of its health. This explosion can hit for nearly 5k health, causing obscene burst damage in combination with normal attacks.
   The best way to deal with this versatile monster is to control it. Unlike most undead, it is susceptible to fear effects as well as anti-undead spell like Shackle Undead and Turn Evil. You can use such skills to buy yourself time to engage the Death Knight on his own. Remember if you have time to kill a DK's pet, you have too much time. DK's can kill you with ease without their pets with any spec because they have skills to counter every class. The only thing about kill a DK is that it takes a lot of effort to kill them while it doesn't take a lot of effort for them to kill you.