Oni

An oni as ally is a deal with a fiend dressed in human skin. The party knows what it is, the oni knows the party knows, and the bargain holds because both sides have something the other wants. Maybe the oni has a grudge against a hag coven the party also hunts. Maybe a warlock PC's patron sent the oni as on-loan muscle. Make the terms ugly and explicit: the oni feeds on someone over the course of the alliance, and the party negotiates on whom and how often.

In combat, the oni runs the disguise for the party's benefit. Shape-Shift lets it impersonate a noble's bodyguard, a hostile cult member, a guard at the gate the party needs to pass. The oni walks the party into the throne room, drops the form on cue, and opens with Charm Person on the duke. From there, it can Multiattack with Claws at +7 for 19 average per hit, drop Darkness as a screen for the rogue's escape, or Bonus Action Invisibility to disappear with the captured target.

Out of combat, the oni is a wonderful liar (Deception +8) and an honest sadist. It tells the truth about its hunger because it is proud of what it is, and it tells lies about everything else. The spell list is short (Charm Person, Darkness, Gaseous Form, and Sleep, all 1/day each), so the oni is not a wand of utility. Use it for the in-person infiltration job and nothing else.

When the bargain ends, the oni collects whatever was promised and walks out wearing a face the party did not know it could wear. A child smiles in the doorway and waves goodbye. The party has a long second to wonder what they just signed.

An oni is a fiend that lives in the shape of someone the party already trusts. Shape-Shift turns it into a Small or Medium Humanoid or a Large Giant, and the oni uses that to walk into a village, befriend a family, eat a child a month, and leave when suspicion rises. Combat begins when the disguise drops, and the disguise should drop on the oni's terms, not the party's.

When the fight opens, lead with Charm Person (level 2 version, two targets) on the strongest melee characters to split the party. If the casters are clustered, swap to Sleep or Darkness centered on the oni itself, then use Bonus Action Invisibility on the next turn for total concealment in the dark cloud. From there, Multiattack gives two Claws at +7 reach 10 ft. for 19 average per hit (10 slashing plus 9 necrotic), or two Nightmare Rays at +5 range 60 ft. for 9 psychic and Frightened until the oni's next turn.

Regeneration 10/turn is the closer. Without a way to spike damage above 10 a round, the oni climbs back to full while the party widdles at it. The hover speed (fly 30) lets the oni stay above melee reach if the room has a ceiling. The party that wins this fight burns through Charm and Invisibility first, then drops the oni in one round before regen catches up. AC 17 and 119 HP supports a real grind if they cannot.

Oni do not die for principle. At Bloodied with spells spent, the oni casts Gaseous Form and seeps through a window or under a door. They will return in three weeks, in a new face, and they will eat the player who hit them hardest first. Make the second visit personal.

Reveal the disguise mid-meal, mid-sentence, mid-blessing. The oni's true face appearing in the middle of an ordinary scene does more work than any combat opener.