Yōki

Yōki (妖気, demonic energy) is the energy used by some characters in the InuYasha Universe. In InuYasha, it’s referring to the energies and power of yōkai. By releasing their yōki energy, some characters like InuYasha or Sesshōmaru can slay hundreds of enemies with one swing of their swords.

Yōketsu
Yōketsu (妖穴, yōki vortex) - visually it manifests in the form of a whirlpool behind demon-like characters that are visible to just a few. This vortex can be cut and it effect an instant-kill upon the demon who's yōketsu is being cut.