Immutable Form. Yellow Devi is immune to any spell or effect that would alter its form.

Magic Resistance. The Yellow Devi has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Yellow Devil's weapon attacks are magical.

Multiattack. The Yellow Devi makes two melee attacks.

Bludgeon. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 7) bludgeoning damage.

Optic Ray.  The Yellow Devil creates three energy rays. It can hurl them at one target or several within 30 ft range. Make a ranged weapon attack for each ray (+13 to hit). On a hit, the target takes 14 (2d6+7) force damage.

Particle Movement (Recharge 5–6). 

The Yellow Devil divides itself in 28 parts that float 30 ft in a straight line to a point it can see. Each creature in that area It passes through must make a DC 19 Dexterity saving throw, taking 63 (14d8) bludgeoning damage on a failed save, or half as much damage on a successful one.

The Yellow Devil can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Yellow Devil regains spent legendary actions at the start of its turn.

Bludgeon. The Yellow Devil makes a bludgeon attack.

Move. The Yellow Devil moves up to it's speed

Particle Expansion (Costs 2 Actions) (Recharge 5–6). The Yellow Devil divides in 28 parts that shot in random directions from its starting position. Each creature within 30 feet of the Yellow Devil must succeed on a DC 19 Dexterity saving throw or take 16 (2d8 + 7) damage on a failed save, or half as much damage on a successful one. The Yellow Devil can reform itself at any point within of the area affected by the attack. This attack cannot pass through full cover.