The Sap (気迫, Kihaku) or Leech(European version) is a type of condition in which it involves at least two or more units.
The effect may inflicted to any unit.
Sap is a condition pertained with two or more units. When a unit performs an action that can Sap, they stick onto the unit, sucking their blood in action. Clitelusses' ability, Blood Drain, is an action that allow it to stick and drain the victim's life. Simply put, the condition allows the sapper to drain HP and the victim to Stagger continuously until the sapper tires out or full.
When Sapping, the sapper and victim's SP are also gradually drained.
If both or any one between the sapper and victim's SP are completely drained due to Sap, they will be inflicted with Lethargy.
When a unit is Sapping, they will stick onto their target victim. Simply put, it's like an action when someone or something is sticking its mouth onto a subject to suck their blood.