Siphon Blood

Siphon Blood (formerly named Blood Siphon) is a skill in Diablo III.

In-game
This channeled skill drains blood from the targeted enemy. It deals 300% damage as Physical, generates Essence and heals the Necromancer each attack cycle. It hits instantly and lasts until the Necromancer stops channeling or the target is out of range. It can drain blood over other enemies, but not through impassable solid obstacles. The effect stops if the target becomes untargetable, immune, invisible etc.

Note that Essence is only generated if Siphon Blood actually deals damage. Since the healing percentage and Essence gain is fixed per tick, faster Attack Speed actually allows healing up very quickly, making faster weapons more preferable for this skill.

Also note that Siphon Blood does not count as a missile since the effect does not travel further and instead relies on the space between the Necromancer and the enemy or designated location. It therefore bypasses Missile Dampening and Slow Time spheres.

Hitting the ground will not extend the spell any further (making firing blindly useless). Hovering the cursor over the target, then holding the left mouse button causes the Necromancer to continuously drain that target, even if the cursor is no longer aimed at that enemy, until LMB is released.

Runes

 * Blood Sucker: Also pulls in all health globes within yards while siphoning.
 * Suppress: Damage type changes to Cold and the target is also chilled by.
 * Power Shift: Damage type changes to Poison, and damage of Siphon Blood is increased by each time it deals damage, stacking up to  times. Stacks last only  seconds, refreshed with each cast.
 * Purity of Essence: Essence gained is increased to per tick while the Necromancer is at full health.
 * Drain Life: Increases the amount of health restored to per second, but no longer restores Essence.

Non-rune enhancements

 * Depth Diggers (Legendary Pants): damage increases by 80-100%.
 * Funerary Pick (Legendary Scythe): drains blood from 2 additional targets.
 * Iron Rose (Legendary Phylactery): channeling Siphon Blood casts Blood Nova on every tick of damage.
 * Trag'Oul's Avatar Set (Set Bonus for 4 items): while at full Life, healing from skills is added to maximum Life for 45 seconds, up to 100% additional Life.
 * Trag'Oul's Avatar Set (Set Bonus for 6 items): healing from skills is increased by 100%.
 * Simplicity's Strength (Legendary Gem): increases damage by 25% (+0.5% per rank) and heals the character for extra 4% of maximum Life per tick (rank 25 bonus).
 * Boyarsky's Chip (Legendary Gem): Taunts the first enemy hit for 2 seconds (rank 25 bonus).
 * Mantle of Channeling (Legendary Pauldrons): while channeling Siphon Blood, increases damage done by 20-25% and reduces damage taken by 25%.

Passives

 * Swift Harvesting: increases attack speed by 15%.

Trivia

 * Drain life is the name of a warlock skill in the Warcraft universe, while purity of essence is the name of a concept in the StarCraft universe.
 * It is the only channeled Primary skill in game (Electrocute is not actually channeled).
 * Regardless of the enemy blood color (or the fact if they even have it at all), the spell does not change its default color of the flow.
 * The Necromancer will alternate hands when channeling this skill, instead of simply using only one or both.
 * Even though it's a channeled skill, it will not be empowered by Taeguk.