Oni are special among giantkind in that they are talented spellcasters and shapeshifters. They walk the line between brute and hag, making for a villain potentially accomplished as both.
Some hags might employ oni as minions, but to be honest, given their intelligence and accomplishments, any service to someone might be temporary at best. Dangerous, cunning and strong villains are rarely anyone’s minions for long, usually having plans of their own and no small amount of ambition.
The oni might be more dangerous when in the service of an even more powerful villain like a dragon, greater devil or demon, lich or mummy lord. This would give the oni access to greater resources, perhaps a helpful organization or even their own pool of agents like cultists or spies.
For the purpose of this encounter, the oni is the agent of some greater villain in your world. I’ve used medusae, succubi and more in this fashion, and they’re all great in their own way. An oni, like a succubus, can be anyone, anywhere, and really ramp up the paranoia. The oni could be hidden among a tribe of orcs, then disguised as a human guard and then as a wandering monk. One never knows.
The Change Shape action states the entire appearance changes size if needed, and their glaive changes in size with them, meaning they’ll always have a polearm. That makes it essential for you to place the oni in roles where the existence of such weapons would be logical and acceptable. You may even wish to make such weapons more common in local soldiers and among peasant militia.
This is supported in historical fact with such weapons as the bill and the military fork. You could even give the oni in question a magical glaive with the ability to temporarily change shape, or give it an ability to call the weapon from anywhere via some sort of teleportation.
There are lots of ways around the predicted metagaming of the experienced and cunning player.
In this particular case, the oni is ready to greet the adventurers as they return to civilization — either ahead of them as they arrive in a new town — or to welcome them as they return to a home community. Just remember, the oni is an intelligent opponent. They act with cunning and a decent tactical sense. They retreat if circumstances take a bad turn and strike when they have some sort of advantage. That’s what makes them deadly.
Encounter Type: Social or combat
Map: Gate encounter map
Lighting: None (flat battle map.)
System: 5E
Length: One shot
Installation Type: Addon
License: Roll20 End User License Agreement
(Personal Use Only, Do Not Distribute)