Omi

Oni appear in Beacon Hills during the latter half of Teen Wolf Season 3.

Appearance and Abilities
They wear modified shinobi shōzoku and black masks similar to Japanese Hannya mask.

Oni appear to be made of black smoke or shadow. They can phase through solid matter, are skilled in martial arts combat, and fight with Ninjatō or straight swords.

Their eyes glow a golden-green color similar to the fireflies seen exiting the Nemeton in More Bad Than Good.

Motive
The Oni are apparently a force for good, seeking only to identify and destroy the Nogitsune but willing to kill anyone who gets in the way of their mission.

Their goal thus far has been to subdue and "tag" each of the supernatural beings (Werewolves, Banshee, Kitsune) with the Japanese symbol for self (Jiko). It appears to be a backwards number "5" behind their left ear. This indicates that the individual is themselves and not possessed by Nogitsune.

Chris Argent has a broken version of their Hannya in a box in his apartment. He believes they have come looking for him because of a prior encounter in which he shot one of them, breaking the mask, when he was 18.

Kira's Mom seems to have some control over the Oni (see Riddled).