Corruption

Corruption is a Shadow-based warlock spell in the Affliction school that damages over time. This spell receives 120% of your spell power from gear and buffs.

Table
Corruption is available at level 4 for.

Improvements
Improved Corruption provides Corruption with a potentially significant bonus to damage - up to 10% at Rank 5. Note: Prior to Patch 3.0.2, this talent reduced the cast time of Corruption. The spell is now instant cast by default, even with no talent points invested.

The Nightfall talent depends on Corruption or Drain Life, giving the potential for an instant Shadowbolt. The Glyph of Corruption has the same effect, except it solely relies on Corruption.

Shadow Mastery will increase damage dealt by Corruption by 15% at max rank, Contagion by 5%, and Demonic Knowledge will increase damage depending on the pet you currently have out. Empowered Corruption increases the bonus damage coefficient from your spell damage bonus by 12% per point, and Everlasting Affliction adds an additional 6% of your spell damage bonus per point (1% per point per tick). Maxing out these two talents will increase the overall spell power coefficient to 120% base + 36% Empowered Corruption + 30% Everlasting Affliction = 186% total.

Pandemic allows the periodic damage to critically hit. Malediction increases the chance of these critical hits.

Corruption is an Affliction spell, and thus gains the benefits of Suppression, Improved Drain Soul, and Grim Reach talents.

Siphon Life adds a self-healing effect to Corruption, and also increases the damage. (Prior to Patch 3.1.0, this talent created its own DoT/HoT ability.)

Glyph of Quick Decay is a Major Glyph that was added to the game in Patch 3.3.0. It decreases the time between the damage ticks of Corruption by an amount equal to your haste. With this Glyph active the warlock talent Eradication also decreases the time between Corruption damage ticks, even though Eradication does not specifically say it increases haste. This Glyph does not increase the number of DoT ticks, but rather decreases the amount of time Corruption takes to deal its full duration of damage. In this way the time between Corruption ticks is reduced as stated by the Glyph.

Tips and tactics
Corruption is considered a bread and butter 'staple' ability of warlocks. When supported with talents, it is quite mana efficient; it provides 2.00 dmg per mana in its level 77 incarnation, or up to 2.20 dmg per mana with the damage bonus from Improved Corruption factored in.

Corruption is usually preferable to Curse of Agony in short fights. This is because Corruption's damage is spread evenly over its duration, and its duration is only 75% that of Curse of Agony (18 seconds vs. 24 seconds). While CoA is more mana efficient (4.52 dmg per mana at max level), that efficiency is lost if the target dies before CoA completes its cycle (especially since CoA deals 50% of its damage in the last one-third of its cycle).

Corruption can be cast with your target behind you, and since it is instant cast it can also be cast while running. As such, this ability is excellent for kiting enemies. Since walking backward is much slower than running forward or strafe-running, it is always best to do one of the latter two things while attempting to kite something.