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Although the skill description mentions saccing "10% health", it acts as though it sacrifices "10% maximum HP", so non-max health doesn't mean proportionally lower sacrificing.
This is one of the fastest casting and fastest recharging offensive skills in the game, and with a high Blood Magic attribute it can be very effective as a means to chip away at an enemy's health, especially interspersed with other skills such as Vampiric Gaze or Barbed Signet. It is a very effective "filler" spell for those times when your other skills are recharging and you don't want to resort to simply attacking. Just make sure not to cast it too often that you endanger your own health.
For this reason, this skill is a very effective one to bring along to fight the Doppelganger; if the monster uses it quickly and often enough, the health it sacrifices will far outweigh the damage you could do to it alone.