Nullicorn

Magic Evasion. If it is subjected to a spell or magical effect that allows it to make a save to take only half damage it instead takes no damage if it makes the save and only half damage if it fails.

Limited Magic Immunity. At the end of its turn any spell or magical effect with duration that is affecting the nullicorn such as hold monster or mage armor immediately ends.

Magic Resistance. Advantage: spell/magic effect saves.

Nullifying Blight. A creature infected with this disease manifests symptoms 1d4 hrs after infection: fatigue light-headedness and blurry vision. When infected creature casts a spell or activates a magic item itchy rash appears and spreads as more magic is used. Until disease is cured when infected creature starts its turn wielding or wearing magic item and each time it casts a spell or activates a magic item it loses 5 (2d4) hp. At end of each long rest infected creature must make a DC 13 Con save. Fail: hp the creature loses each time it uses magic increases by 2 (1d4). A creature that makes 3 saves recovers.

Sense Magic. Senses magic within 120' of it at will. This trait otherwise works like the detect magic spell but isn't itself magical.

Multiattack. Two Hooves attacks and one Void Horn attack.

Hooves. Melee Weapon Attack: +7 to hit, 5 ft., one target, 11 (2d6+4) bludgeoning damage.

Void Horn. Melee Weapon Attack: +7 to hit, 5 ft., one target, 8 (1d8+4) force and target must make DC 13 Con save or contract the nullifying blight disease (above).

Ripple of darkness. Each creature within 20' of the nullicorn: 18 (4d8) necrotic and contracts the nullifying blight disease (above; DC 13 Con half and doesn't contract it).