Every time you kill an enemy using spells, you will receive a certain amount of SP back, depending on the monster level.
The formula is MonsterLV x Drain. The Drain amount is (95+15*SkillLV)% (e.g. killing a LV 20 Enemy with a LV 5 Soul Drain will give you 34 SP). This skill also increases your Maximum SP by 2% per SkillLV.
The SP drain only works for targeted spells, not area spells.

Increases MaxSP.
Whenever you defeat an enemy with magical or physical attack, you will gain certain amount of SP depend on monster's level.