The Lightning Capacitor

The Lightning Capacitor is an epic trinket for DPS casters.For some classes and specs it was said to be the most powerful trinket in the game.

Source
This item drops from Terestian Illhoof in Karazhan.

Additional notes

 * You do not need to face the mob, which is supposed to be hit by the bolt.
 * The caused damage by the Lightning Bolt is unaffected by the characters spell power, which can be explained by the fact, thet the Lightning Bolt might appear like an instant cast spell, but it does not trigger the Global Cooldown and hence it's damaging coefficient is 0 / 3.5 = 0. Firing a Lightning Bolt also does not interrupt channeled casts like Arcane Missiles.
 * The Lightning Bolt does not have a max-range, as long as you can crit the mob, the Bolt can reach it. The Bolt is also unaffected by LoS-mechanics. Think of the following situation: You have 2 charges and you cast a fireball on a mob, while the fireball is in mid-air, the mob runs behind a wall or some LoS blocking obstacle. The fireball crits, giving the 3rd charge and thus releases the bolt; even though the mob is not hittable by your spells, the bolt will fly right through the obstacle and hit the mob. The same applies to silencing effects, stuns and other incapacitating effects, basically there is nothing that could prevent The Lightning Capacitor from firing it's Bolt when reaching 3 charges, by any available means, except the case that the mob died from the 3rd crit, then the charges are lost and no Bolt is fired.
 * The Lightning Bolt always hits the mob which received the last crit that stacked the 3rd charge and released the Bolt, even though you may not have targeted it, except it died from the crit, as mentioned above.
 * The Electrical Charges are displayed as a stackable, typeless buff on you. This buff has an infinite duration and even persists if you unequip The Lightning Capacitor or logout. It does not persist through death, however, and like other buffs, it disappears upon entering an Arena match.
 * It can proc from any ability that is considered a spell, which includes Flametongue Weapon, Frostbrand Weapon, Lightning Overload-casts, Firestone (item) attack and Molten Armor. Wand crits are not considered spell crits and hence do not produce charges.
 * The damage of the Lightning Bolt is affected by debuffs (e.g. Stormstrike) and talents (e.g.Molten Fury (mage talent)) which improve all, or at least nature spells. However, talents which specifically mention their affected spells do not improve it. Furthermore, even the Shaman's talents which improve Lightning Bolt do not affect it, as these only affect the Shaman's spell; these include Concussion, Elemental Mastery and Elemental Fury. Especially in raid environments, where many buffs and debuffs are in effect, the dps boost provided is slightly higher (~80 dps), especially with spell crit increasing auras, totems or buffs.
 * The proc of this trinkets appears in your combat log as "Your Lightning Bolt hits/crits %t for %d.", just like the Lightning Bolts fired by Shamans.