Horrify effect
Horrify effects or horror effects are a form of crowd control. Horrify effects may root the target or cause them to run in a random direction, but all prevent the target from taking most actions for the duration.
Horrify effects are magical debuffs, and can therefore be dispelled. Horrify effects in the Shadow school can also be resisted. Horrify effects are subject to diminishing returns.
Abilities that cause the horrify effect
[Mortal Coil] (warlock) causes the target to run in horror for 3 seconds, and restores 15% of the caster's maximum health. 45 second cooldown.
[Blood Horror] (warlock) causes the next damaging melee strike against the warlock to send the enemy running in horror for 4 seconds. 30 sec cooldown.
[Psychic Horror] (Shadow priest) roots and horrifies the target for 2–4 seconds (depending on the number of 
[Shadow Orbs] consumed), and also disarms them for 10 seconds. 45 second cooldown.- Priest PvP Satin set bonus: When your Vampiric Touch is dispelled, the dispeller and all nearby enemy targets within 6 yards are instantly feared in horror for 3 sec.
Fittingly, horrify effects are therefore limited to warlocks and Shadow priests.
Horrify effects have their own Horror category of diminishing returns. However, 
[Sin and Punishment] is not subject to diminishing returns.
Non-player abilities
Some mobs also use horrify effects, such as Fen Rays in the Underbog, or Brienna Nightfell/Vivienne Blackwhisper in Trial of the Crusader.
Countering horrify effects
- The item
[Nimble Brew], created by the Brewmaster monk ability [Craft: Nimble Brew], specifically removes horror effects. - Like all forms of crowd control, horrify effects can be removed with the PvP trinkets or the human racial
[Will to Survive]. - Immunities such as
[Ice Block] and 
[Divine Shield] will prevent and remove any horrify effects currently afflicting the caster. - Although similar to fear effects, horrify effects are classified separately and cannot be removed with abilities and effects that affect fear, such as
[Berserker Rage] or 
[Will of the Forsaken]. Compared to the large range of counters available for fear effects, this makes horrify effects extremely hard to counter. - All current player horrify effects are magical debuffs, and can therefore be dispelled by druids, paladins, priests, shamans and the Imp warlock minion.
- All current player horrify effects belong to the Shadow school, and can therefore be resisted.
- Most creatures that are immune to fear effects are also immune to horrify effects. This includes most Undead and Mechanicals, as well as most Bosses.
Patch changes
Patch 5.4.0 (2013-09-10): All fear and horrify effects can no longer cause targets under the effects of a movement speed buff to move faster than normal run speed. - Patch 5.2.0 (2013-03-05): [Blood Horror] added, replacing
[Blood Fear]. - Patch 5.0.4 (2012-08-28): [Sin and Punishment] changed from talent to PvP Season 12 Satin set bonus.
[Death Coil] changed to [Mortal Coil]. [Psychic Horror] altered. 
Patch 4.2.0 (2011-06-28): [Sin and Punishment]'s backlash fear effect no longer has diminishing returns. - Hotfix (2011-01-18): Sin and Punishment no longer shares diminishing returns with fear effects. It now shares diminishing returns with horrify effects.
Patch 3.1.0 (2009-04-14): [Psychic Horror] has been redesigned and is now considered a Horrify effect. 
Patch 2.1.0 (2007-05-22): [Death Coil] is now subject to diminishing returns in the Horror category. Currently, only Death Coil is in this category.