Large giant, lawful evil

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

At Will: darkness, invisibility, hex

1/day each: charm person, cone of cold, gaseous form, bestow curse, contagion

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

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

Actions

Multiattack. The oni makes two attacks: one with its cursed glaive and one with its goring horn.

Cursed Glaive. Melee Weapon Attack: +7 to hit, reach 10ft., one target. Hit ; 17 (2d12 + 4) slashing damage, or 10 (1d12 +4) slashing damage in small or medium form. The target must then make a DC 15 Constitution saving throw, on a fail they are branded by the curse. The oni gains advantage on attack rolls against branded targets. They may repeat this saving throw at the end of each of their turns.

Goring Horn. Melee Weapon Attack: +7 to hit, reach 5ft., one target. Hit ; 15 (2d10 + 4) piercing damage, or 9 (1d10 + 4) piercing damage in small or medium form.

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 to its normal size.