Charge. If the unicorn moves at least 20 feet straight toward a target and then hits it with a horn attack on the same turn, the target takes an extra 9 (2d8) piercing damage. If the target is a creature, it must succeed on a DC 15 Strength saving throw or be knocked prone.

Innate Spellcasting. The unicorn's innate spellcasting ability is Charisma (spell save DC 17). The unicorn can innately cast the following spells, requiring no components:

At will: detect evil and good, druidcraft, pass without trace

1/day each: calm emotions, dispel evil and good, entangle

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

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

Unarmored Defense

When unicorns are not wearing barding  their AC equals 10 + your Dexterity modifier + your Wisdom modifier.

Total Equine Control (3/day)

They can take the Disengage, Dodge or Dash action as a bonus action on its turn, and it's jump distance is doubled for the turn.

Improved Spellcasting. The unicorn's cast spells as a 3rd level sorcerer, it's spellcasting ability is Charisma (spell save DC 15). 

Cantrips- Shocking Grasp, Control Flame, Message, Ray of Frost

1st - (4 per day)- Thunderwave, Shield, Mage Armor, Color Spray

2nd - (2 per day) See Invisibility, Web

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Multiattack. The unicorn makes two attacks: one with its hooves and one with its horn.

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

Horn. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage.

Healing Touch (3/Day). The unicorn touches another creature with its horn. The target magically regains 11 (2d8 + 2) hit points. In addition, the touch removes all diseases and neutralizes all poisons afflicting the target.

Teleport (1/Day). The unicorn magically teleports itself and up to three willing creatures it can see within 5 feet of it, along with any equipment they are wearing or carrying, to a location the unicorn is familiar with, up to 1 mile away.

Unicorn Fighting Style: When they  use the MultiAttack action with horn and hoof on its turn, they can make one unarmed strike as a bonus action (1d4+4).

Thunder Charge (3/day):  As a bonus action during the unicorns charge, each creature along it's path is subject to the Thunderwave spell (DC 15 CON).  If the creatures fails their save, they are pushed back as per spell and cannot make an opportunity attack.

Rainbow Horn:  If the unicorn hits an opponent by more than 5 with it's horn attack, it can cast Color Spray as a bonus action centered on the creature hit

The unicorn 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 unicorn regains spent legendary actions at the start of its turn.

Hooves. The unicorn makes one attack with its hooves.

Shimmering Shield (Costs 2 Actions). The unicorn creates a shimmering, magical field around itself or another creature it can see within 60 feet of it. The target gains a +2 bonus to AC until the end of the unicorn's next turn.

Heal Self (Costs 3 Actions). The unicorn magically regains 11 (2d8 + 2) hit points.