Aitken

Rampage. When the Wild Mange reduces a creature to 0 hit points with a melee attack on its turn, the Wild Mange can take a bonus action to move up to half its speed and make a bite attack.

Shapechanger. The Wild Mange can use its action to polymorph into a creature-humanoid hybrid or back into its true form, which is humanoid. Its statistics, other than its AC, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Curse of the Wild Mange. When a spellcaster is under the effects of Wild Mange lycanthropy, each spell they cast while cursed triggers a Wild Magic Surge. See the Sorcerer's Wild Magic table in the PHB.

Metamagic. Aitken can bend the wild magic within him. He has 14 sorcery points and can do the following:
Quickened Spell: When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.
Transmuted Spell: When you cast a spell that deals a type of damage from the following list, you can spend 1 sorcery point to change that damage type to one of the other listed types: acid, cold, fire, lightning, poison, thunder.
Heightened Spell: When you cast a spell that forces a creature to make a saving throw to resist its effects, you can spend 3 sorcery points to give one target of the spell disadvantage on its first saving throw made against the spell.

Spellcasting. Aitken is a 14-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). Aitken has following Sorcerer spells prepared:
Cantrips (at will): booming blade, chill touch, fire bolt, message, mind sliver, shocking grasp 
1st level (4 slots): absorb elements, chaos bolt 
2nd level (3 slots): hold person, misty step
3rd level (3 slots): counterspell, fireball, thunder step
4th level (3 slots): dimension door, wall of fire
5th level (2 slots): cone of cold, teleportation circle
6th level (1 slot): chain lightning
7th level (1 slot): teleport

Mad Multiattack. Roll 1d4 at the beginning of each of the Wild Mange's turns to determine how many attacks it can make each round. Using this pool of actions the Wild Mange can attack in the following ways:

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 6 (2d6 + 3) piercing damage. If the target is a humanoid, it must succeed on a DC 12 Constitution saving throw or be cursed with Wild Mange lycanthropy.

Spell-Scythe. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 8 (1d12 + 3) slashing damage. On a hit, the Wild Mange drains one spell slot of the targets lowest level available and regains 9 (2d4+4) hit points.

Scintillating Strobe. The Wild Mange emits a blinding array of colors. Any creature  within 5 ft. of the creature must succeed on a DC 15 Wisdom saving throw or become stunned until the end of their next turn.

Chaotic Crescendo. When the Wild Mange is reduced to 0 hit points, it explodes. Each creature within 15 feet of the Wild Mange must make a DC 15 Dexterity saving throw, taking 13 (2d12) lightning damage on a failed save, or half as much damage on a successful one. When the Wild Mange is reduced to 0 hit points roll once on the Sorcerer's Wild Magic table and proceed with the following effect.

Aitken can take 2 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. Aitken regains spent legendary actions at the start of its turn. 

Shock Step. The Wild Mange teleports up to 60 feet. Immediately after it disappears, the tear in reality bursts, and each creature within 10 feet of the space the Wild Mange formerly occupied must make a DC 15 Constitution saving throw, taking 6 (1d12) lightning damage on a failed save, or half as much damage on a successful one.