Yōki

The is the  energy and power utilized by yōkai. By releasing their yōki energy, some, such as Inuyasha or Sesshōmaru can slay hundreds of enemies with one swing of their swords.

Yōketsu
is the visual manifestation of demonic energy in the form of a whirlpool that appears behind a yōkai and is visible to just a few. This vortex can be cut and its effect is an instant-kill upon the demon whose yōketsu is being cut; Inuyasha's Dragon-scaled Tessaiga had this ability.