Swallow Soul is a skill located in the Siphoning (which can be found in the Nightblade skill tree). Swallow Soul is a morph of Strife.
Swallow Soul deals the following types of damage: Magic Damage. Which means that it gets buffed by the following blue champion point trees: Blessed, Elemental Expert, Elf Born, Master at Arms, Quick Recovery, Spell Erosion. Also the following red champion point trees protect against this skill: Elemental Defender, Iron Clad.