Death Knights are incredibly strong at leveling in Wrath of the Lich King due to their self sustainability and very high damage output, as well as being able to maintain a very high speed of slaying large packs of monsters. Being able to Death Strike as a way to self heal and wearing Plate armor, Death Knights have very minimal downtime during the leveling process. Combined with the two Unholy talents On a Pale Horse and Improved Unholy Presence, Death Knights move throughout the world while mounted as well as on foot much faster than other classes.
Leveling as Unholy is what we recommend. Unholy can do what Blood and Frost can do, but also comes with extra bonuses such as the movement speed perks and it gives the highest AoE DPS output of all 3 specializations for leveling, while still maintaining a very high self healing capability. You will want to always engage between 2-6 monsters at a time (this can vary to even more than 6 depending on your gear level).
The main priority is keeping up your diseases on all targets at all times using your Frost and Unholy runes. Using Plague Strike and Icy Touch will apply your 2 DoT diseases, Frost Fever and Blood Plague. Due to being specialized deep into the Unholy tree your diseases will also apply a third non DoT disease Ebon Plaguebringer which serves as a damage buff on all monsters affected by any of your diseases. Once your diseases have been applied, using a Blood rune to cast Pestilence on that monster will spread those diseases to all enemies within 10 yards.
This now gives us the chance to cast Death and Decay which is a very powerful damaging ability on the ground which lasts 10 seconds. Be sure to place it cautiously as it is very costly to cast and once placed make sure the monsters remain inside the circle area so they take damage from its effect over its entire duration.
As your runes regenerate you can use Frost and Unholy runes on Death Strike to heal. Remember that Death Strike only heals you while you have diseases on the enemy! Blood runes can be used to cast Blood Boil which will deal additional AoE damage if you’re still fighting a pack of monsters, otherwise Blood Strike on single target.
Runic Power should be used on Death Coil as well as Rune Strike for when it procs. When fighting elite monsters having saved 60 Runic Power before engaging is ideal so you can instantly summon your Gargoyle.
Casting Corpse Explosion should be prioritized when fighting large packs of monsters. You can use a macro
/cast [@pet] Corpse Explosion which will make your pet explode dealing massive amounts of damage to all enemies around your pet (this kills your pet but resummoning it only has a 30 seconds cooldown). Once a regular monster dies you can also use Corpse Explosion on its corpse. You should prioritize killing low health monsters when AoEing so you can use Corpse Explosion sooner to deal damage to surrounding enemies. Note: Exploding your pet deals more damage than exploding the corpse of a regular monster.
When fighting in single target scenarios Ghoul Frenzy should be used to buff the damage of your Ghoul. Empowered Rune Weapon is also a very strong 5 minute cooldown which instantly resets the cooldown of all your runes. This can be used in emergencies when needing to Death Strike or use other abilities.
We recommend you use a 2 Handed weapon while leveling as Unholy until you’ve reached max level. Your weapon runeforge enchant should be either Razorice or Cinderglacier initially. If you have the option of using Crusader however you should use that until you’re level 70 and then switch to Fallen Crusader at that point. Blood Presence should always be used when leveling. If wanting to tank a dungeon this Unholy spec is completely fine just switch to Frost Presence for the dungeon.
Keep in mind when AoEing not to spend all your Runic Power on Rune Strike or Death Coil. Corpse Explosion should be used when AoEing which also costs 40 Runic Power to cast.