Wikia

Devouring Plague
Devouring Plague 30 yd range None/GCD cooldown 25% of base mana Instant cast Afflicts the target with a disease that causes X Shadow damage over 24 sec. 15% of damage caused by the Devouring Plague heals the caster. This spell can only affect one target at a time.
Usable by
Class	Priest
Properties
Type	Offensive
School	Shadow
Cooldown	None/GCD
Other information
Ranks	7
Related debuff
Disease Devouring Plague Causes {X/8} damage every 3 seconds, healing the caster. Duration: 24 seconds

"When the shadows are through with you, there will be nothing left, not even your bones."
—Deacon Markus Hallow^[1]

Devouring Plague is a Shadow-based Life Drain Damage over Time spell, which heals the caster for 15% of the damage done.

Rank table

Rank	Damage	Level	Cost
1	152	20	30
2	272	28	80
3	400	36	140
4	544	44	240
5	712	52	380
6	904	60	460
7	1088	68	11
8	1144	73	18
9	1376	79	18

Shadow tree talents:

[Darkness], increases the Shadow based damage by 2/4/6/8/10%.
[Shadow Affinity], reduces threat by 8/16/25%.
[Shadow Focus], increases chance to hit by 1/2/3% and reduces the mana cost by 2/4/6%.
[Shadow Reach], increases the range by 10/20%.
Passive: [Shadow Weaving], is a Shadow damage incremental buff. Damage is increased by 2/4/6/8/10% (note: Instant cast spells cannot apply this debuff).
Passive: [Vampiric Embrace], if a target is afflicted with this spell, Devouring Plague will additionaly heal the Priest for 15/20/25% of damage done ([Improved Vampiric Embrace] included). Total healing received is 30/35/40% of damage done.
[Mind Melt], increases the critical strike chance of periodic ticks by 3/6%.
[Improved Devouring Plague], increases periodic damage by 5/10/15% and adds a direct damage effect for 10/20/30% of the total periodic damage, for a net damage increase of 15.5/32/49.5%.
[Shadowform], increases the damage done by 15%, and reduces threat by 30%, and gives Devouring Plague (among other DoTs) to have a 100% increased critical strike damage bonus.
[Misery], increases chance to hit by 1/2/3% (triggered by Shadow Word: Pain, Vampiric Touch and Mind Flay)

Discipline tree talents:

[Twin Disciplines], increases the healing and damage done by 1/2/3/4/5% by all instant cast spells.
[Silent Resolve], reduces dispel chance by 10/20/30%.
[Mental Agility], reduces the mana cost of all instant casts by 3/6/10%.

If you are Shadow specced, you can deal an immense amount of damage with just Shadow Word: Pain and Devouring Plague.
Devouring Plague cannot be dispelled by a Warlock's felhunter, which makes it an interesting option in duels..

It will do extra healing if you buff Vampiric Embrace.
It is worth mentioning that this spell is classified as a Disease, not as a magical debuff.

Patch 3.2.0 (04-Aug-2009): Cooldown removed but may only be inflicted upon a single target.
Patch 3.1.0 (14-Apr-2009): This spell now has a new icon.
/ Patch 3.0.2 (14-Oct-2008): Devouring Plague made available to all priests.
Patch 1.8.0 (10-Oct-2005): Will now properly consume the Inner Focus effect when cast.
Patch 1.6.0 (12-Jul-2005): Health leech effects no longer give you health while you are a ghost.