Unidentified oni

An oni was responsible for a series of missing children incidents in Westruun in an adventure hook in the Tal'Dorei Campaign Setting sourcebook, set in 812 PD.

Description
This oni was sadistic and clever at covering their tracks. Like most oni, it was lawful evil.

History
In addition to the series of missing children incidents, the oni was responsible for a plague of nightmares affecting people of all ages. Magical experts were sought to look into either matter, although the oni was able to cover their tracks well. It's unclear whether the nightmares were also connected to the dream-reading stall recently opened by tiefling twins Nyx and Vryll.

Oni abilities

 * Flying speed of 30 feet per round
 * Darkvision for 60 feet
 * Innate spellcasting: The oni was able to innately cast the following spells without material components.
 * At will:
 * Darkness
 * Invisibility
 * 1/day each:
 * Charm Person
 * Cone of Cold
 * Gaseous Form
 * Sleep
 * Magic weapons: The oni's attacks were magical.
 * Regeneration: The oni regenerated every round as long as they were conscious.
 * Actions:
 * Multiattack: The oni could make two attacks per turn with their claws or glaive.
 * Claw: A melee weapon attack that dealt slashing damage; usable in the oni's true form.
 * Glaive: A melee weapon attack with reach that also dealt slashing damage; usable in either form, albeit weaker in their humanoid disguise.
 * Shape Change: The oni could magically polymorph into a small or medium-sized humanoid, a large-size giant, or back into their true form as an oni. With the exception of its size, their statistics remained the same, and their glaive shrunk to match the humanoid form. Upon death, the oni and glaive would revert to their true form and size, respectively.

Equipment

 * Chainmail armor
 * Glaive