Death Coil (aka skill coil) is a basic Warlock instant cast spell with a cooldown that damages the target as well as gives you some health. In addition, the target would lose control for a few seconds, called a "Horror" effect which is different than "Fear". Death Coil is the Warlock counter to a Warrior who is immune to Fear (Berserker Rage / Deathwish) and a Rogue giving you a few precious seconds to get away from their melee range. Of course, Rogues have their Cloak of Shadows (aka cloak of skill) to render this useless. Death Coil can also be used to interrupt a caster's spellcast, as it is instant. Furthermore, while the target is under the influence of the horror effect, the Warlock can cast Fear and frustrate the target even further, giving the Warlock time to bandage and prepare a counter attack.

Dark Pact is a very powerful Warlock Affliction tree talent. Because Warlocks can already Drain Mana (on mobs/players with mana) and Life Tap (convert his HP to mana), having Dark Pact in addition to the two spells means an Affliction Warlock basically would very rarely find the need to sit down and drink to regen mana. Dark Pact is instant cast with no cooldown and what it does is drains mana from the Warlock's pet to the Warlock himself. In essence, the pet becomes the Warlock's mana battery, which is especially true for the Imp and Succubus due to their large mana pool and fast regen.