Description: Skill. Sacrifice 10% max Health. Target touched foe takes 20..50 shadow damage.
Energy Cost: 5
Activation Time: .75
Recharge Time: 3
Linked Attribute: Blood Magic
Skill Quest: Ephaz - Beetletun

61 damage at level 16, not sure if it's wise to use AtB to raise damage to 66 at the cost of more sacrifice.

Dark Pact is the ranged version of this spell, but there are some reason's to consider Touch of Agony. Most of the time, however, this skill is used always in conjunction with Dark Pact.

The disadvantages of this skill are the 10% sacrifice, which can be viewed as an advantage in a certain build. The other, of course, it the touch-range requirement. A regular life siphoning, vampiric gazing necromancer probably doesn't want to stand right next to enemies to use this skill.

The advantages: .75 cast time (the apparent benchmark for interruption), short recharge, low energy cost, armor-ignoring damage, touch skill cannot be interrupted by most mesmer-type interrupts.

The most popular use of this build is with maxed blood and death magic with the following skillset:
Dark Pact
Touch of Agony
Dark Aura
Aura of the Lich
Blood Renewal

Aura of the Lich reduces the amount of sacrificed health from Dark Pact and Touch of Agony to a very small amount. Using Dark Aura, the caster can stand next to enemies and spam Touch of Agony and Dark Pact for the initial damage as well as an AoE provided by Dark Aura. With a 16/15 blood/death attribute setup, Dark Aura will add 50 damage to adjacent foes everytime you use Dark Pact, Touch of Agony, or Blood Renewal. The primary target takes around 100 damage per pact or touch. Since they're cheap and short-recharged, you can quickly stack on a lot of damage that ignores armor. It's important to watch health carefully and maintain it with Blood Renewal. Heavy degeneration that cancels renewal is troublesome.

Factions will present the duplicate skill Wallow's Bite which might replace Dark Pact in this build for better damage.

A possible addition to the previous build could includes Signet of Agony and Plague Sending. Both are sacrificial and trigger Dark Aura, while you have additional damage and transfer bleeding onto enemies.

"Touch Rangers" can also use this skill with Vampiric Touch to sacrifice then steal back health, energy costs all reduced by Expertise.

The Fog Nightmare species in Kryta and Nightmare species in the Ring of Fire as well as certain bosses of this species use Touch of Agony, almost always with Dark Aura and some sort of healing.

Touch of Agony can be compared to the similar Death Magic version, Vile Touch. Vile Touch costs an additional 5 energy and 1 second longer recharge, but deals slightly more damage while removing the sacrifice effect. Touch of Agony can still be seen as dealing significantly more damage at a cheaper cost with Dark Aura.

Ratings
PvE: 4/10 - I don't think people tend to "dark bomb" here. There's often enchantment removal and powerful melee's that would shred a fragile necromancer.
CA: 7/10 - Tends to be most effective here.
TA: 6/10 - Still effective, hard to stop via interruptions, though effectiveness is dependent on healing and other skills.
Tombs/GvG: 2/10 - Necromancers typically play different roles. Getting into melee to spike with this is very risky. Perhaps secondaries can utilize the skill...