The Lifesteal Fist skill comes from the Ash of War: Lifesteal Fist, which is a drop from a Teadrop Scarab roaming around the Caelid Highway. For more information about how to get this Ash of War ...

"Lifesteal Fist: Skill that demonstrates mastery of the art of controlling vital energies. A slow, controlled punch with an energy-infused fist that renders foes unconscious and steals their HP.