Your Soul Leech effect also restores mana to you and your summoned demon equal to X% of maximum mana, and has a Y% chance to grant up to 10 party or raid members mana regeneration equal to 1% of maximum mana per 5 sec. Lasts for 15 sec.
Improved Soul Leech is a talent in the destruction tree, you will need to spend 40 points in destruction to max it.
As a Destruction Warlock, the only real way to quickly regenerate mana is with Life Tap. Improved Soul Leech 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. Thanks to the Replenishment effect, this talent is very useful for raiding.