Wikia

Unholy Blight
Unholy Blight 1.5 min cooldown Instant cast Surrounds the Death Knight with a vile swarm of unholy insects for 10 sec, stinging all enemies within 10 yards every 1 sec, infecting them with Blood Plague and Frost Fever.
Usable by
Class	Death knight
Properties
School	Shadow
Cooldown	1.5 min (GCD 1 sec)
Talents
Talented	Yes
Level required	56
Related buff
Unholy Blight Surrounded by a vile swarm of unholy insects. Infecting nearby enemies with Blood Plague and Frost Fever every 1 sec. Duration: 10 seconds

Unholy Blight is a Death knight talent available at level 56.

Patch changes

Patch 5.0.4 (28 August 2012): Updated.
Patch 3.3.3 (2010-03-23): In addition to its previous effects, this talent now also prevents diseases from being dispelled from victims afflicted by Unholy Blight.
Patch 3.3.0 (08-Dec-2009): This talent now deals only 10% of Death Coil damage as a damage-over-time effect on the target.
Patch 3.2.0 (04-Aug-2009): This talent has been redesigned. It no longer deals damage to nearby targets. Instead, when you deal damage with Death Coil, the target will take periodic damage for 10 seconds equal to 20% of the damage done by Death Coil. This damage accumulates in the same way as Ignite and Deep Wounds.
Patch 3.1.0 (14-Apr-2009): Have swapped places with Gargoyle in the Unholy tree. Gargoyle is the new Unholy 51 talent.
Patch 3.0.8 (2009-01-20): Cost reduced from 60 to 40 runic power. Will no longer cause spell pushback.