Siphon Life (rank 3)

• Demon SkinLevel 1 · Rank 1
• Shadow BoltLevel 1 · Rank 1
• Summon ImpLevel 1
• CorruptionLevel 4 · Rank 1
• Curse of WeaknessLevel 4 · Rank 1
• Life TapLevel 6 · Rank 1
• Curse of AgonyLevel 8 · Rank 1
• FearLevel 8 · Rank 1
• Create Healthstone (Minor)Level 10
• Drain SoulLevel 10 · Rank 1
• Summon VoidwalkerLevel 10
• Health FunnelLevel 12 · Rank 1
• Curse of RecklessnessLevel 14 · Rank 1
• Drain LifeLevel 14 · Rank 1
• Create Soulstone (Minor)Level 18
• Demon ArmorLevel 20 · Rank 1
• ShadowburnLevel 20 · Rank 1
• Summon SuccubusLevel 20
• Create Healthstone (Lesser)Level 22
• Eye of KilroggLevel 22
• Drain ManaLevel 24 · Rank 1
• Curse of TonguesLevel 26 · Rank 1
• Detect Lesser InvisibilityLevel 26
• BanishLevel 28 · Rank 1
• Create Soulstone (Lesser)Level 30
• Enslave DemonLevel 30 · Rank 1
• Siphon LifeLevel 30 · Rank 1
• Summon FelhunterLevel 30
• Shadow WardLevel 32 · Rank 1
• Create HealthstoneLevel 34
• Create SpellstoneLevel 36
• Detect InvisibilityLevel 38
• Create SoulstoneLevel 40
• Howl of TerrorLevel 40 · Rank 1
• Soul LinkLevel 40
• Death CoilLevel 42 · Rank 1
• Create Healthstone (Greater)Level 46
• Create Spellstone (Greater)Level 48
• Create Soulstone (Greater)Level 50
• Detect Greater InvisibilityLevel 50

And 5 more.

The free addon pack puts it on your bars, or see the whole Warlock spell list.