Sap Essence is a Nightblade Class skill, found in the Siphoning Skill Line.
Base Skill: Drain Power
|Siphon the vigor from your enemies' blood, dealing 1742 Magic Damage to all nearby enemies and healing you and your allies for 599 plus 20% more for each enemy hit. If an enemy is hit, you gain Major Brutality and Sorcery, increasing your Weapon and Spell Damage by 20% for 30 seconds.|
Sap Essence is a morph of the Drain Power base skill. The other morph is Power Extraction.