Corrupted Reindeer

Legendary Resistance (2/day).  If the reindeer fails a saving throw, it can choose to succeed instead.

Charge. If the reindeer moves at least 20 feet straight toward a target and then hits it with a ram attack on the same turn, the target takes an extra 1d10 damage. If the target is a creature, it must succeed on a DC 16 Strength saving throw or be knocked prone.

Flyby. If the reindeer is in the air and moves at least 20 feet straight toward a target and then hits it with a ram attack on the same turn, it doesn't provoke opportunity attacks if he moves out of melee range with the target or any of its allies that are adjacent.

Life drain (2/day). When the reindeer falls below half of its hit point maximum, the Humbug parasite attached to it lashes out at 1 target within 15 feet of him with a proboscis-like appendage. Melee weapon attack: +7 to hit, range 15 ft., 1 target. Hit: 4d10 nectrotic damage, and the reindeer regains hit points equal to double the damage dealt, rounded up.

Multiattack. The reindeer makes one ram attack and one hoof attack.

Ram. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit:  4d8 + 4 piercing damage. 

Hooves. MeleeWeapon Attack: +7 to hit, range 5 ft., 1 target. Hit: 5d6 + 4 bludgeoning damage.

Nose Beam (Recharge 5-6). The reindeer fires a beam of energy from its nose in a 90 foot long, 5 foot wide line. Each creature in this area must make a DC 15  dexterity saving throw, or take 7d10 radiant damage.

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

Ram (Costs 1 Action). The reindeer makes a ram attack with its antlers.

...Had a Ver Shiny Nose (Costs 2 Actions: Recharge 5-6). The reindeer's nose pulses painfully birght. Each creature within 30 feet of it must make a DC 15 constitution saving throw. On a failure, they are blinded for the next minute, or until they succeed on a constitution saving throw at the end of their turn.