From WoWWiki
Improved Soul Leech is a talent in the destruction tree, you will need to spend 40 points in destruction to max it.
Rank table
Notes
As a Destruction Warlock, the only real way to quickly regenerate mana is with Life Tap. Improved Soul Leech just adds a bit more mana that isn't tied into burning health away to get more mana. Keep in mind that this talent also gives your pet mana. Since Destruction Warlocks tend to use the Imp, which has a tendency to run out of mana, this talent allows the Warlock to keep the pet's DPS potential at its highest. With the new Replenishment addition, the talent becomes much more useful for raid utility.
Patch changes
