Innate Spellcasting. The oni's innate spellcasting ability is Charisma (spell save DC 13). The oni can innately cast the following spells, requiring no material components:

At will: misty step, invisibility

1/day each: [spellsuggestion[/spell], cloudkill, polymorph, sleep

Magic Weapons. The oni's weapon attacks are magical.

Regeneration. The oni regains 10 hit points at the start of its turn if it has at least 1 hit point.

Multiattack. The oni makes two attacks, one with its Coin Sweep and one Lever Arm.

Coin Sweep. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) bludgeoning damage. On hit, gold coins (equal to damage taken) spray from the pockets of the victim and heal the One-Armed Oni equal to the damage dealt.

Lever Arm. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) bludgeoning damage, and the target must make a DC 16 Intelligence saving throw. On a failed save, the target's last used spell, magical item, or ability is disabled until the target forfeits half their gold (or valuable item) to the One-Armed Oni as an action, or is dispelled with a Remove Curse spell.

Change Shape. The oni magically polymorphs into a Small or Medium humanoid, into a Large giant, or back into its true form. Other than its size, its statistics are the same in each form. The only equipment that is transformed is its glaive, which shrinks so that it can be wielded in humanoid form. If the oni dies, it reverts to its true form, and its glaive reverts to its normal size.