Bandersnatch

Bandercoat. The fur of the Bandersnatch is an incredibly strange magical substance. It is constantly thriving and changing. At the start of every of the Bandersnatch's turns, it rolls a D20 and a D6. The result of the D20 will be added to its AC for that round. The result of the D6 roll will decide for what elemental damage type it is vulnerable (1=Fire, 2=Cold, 3=Lightning, 4=Acid, 5=Poison, 6=Thunder). It will have resistance to all other damage types. Additionally, every time a creature hits it with a melee weapon attack, the enemy will have to make a DC 18 strength saving throw or have the weapon stuck in the Bandersnatch's coat. The weapon will fall out once the Bandersnatch moves.

Protective spirit. When any creature deals damage to Gullveig, the Bandersnatch's eyes will immediately grow red with anger. It gains a free movement towards the creature that attacked his mistress and can attack it when it reaches it, dealing an additional 20 (2D8 +8) bludgeoning damage. For the following three turns, the Bandersnatch's can attack 6 times in stead of 3 on its turn.

Fey monster. The Bandersnatch's attacks are considered magical for the sake of overcoming resistance. 

Multiattack. The Bandersnatch makes three attacks, one with his jaws, one with his claws and one with his tail. 

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., 1 target. Hit: 19 (2d10 + 8) piercing damage. 
Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., 1 target. Hit: 15 (2d6 + 8) slashing damage. 
Tail. Melee Weapon Attack: +13 to hit, reach 15 ft. Behind, 1 target. Hit: 17 (2d8 + 8) bludgeoning damage. 

The Bandersnatch can take 2 legendary actions choosing from the options below. Only one legendary action can be taken at a time and only at the end of another creatures turn. The Bandersnatch regains any spent legendary actions at the start of its turns.

Bander dash (Costs 1 Actions). The Bandersnatch bounds across the battlefield. All creatures in a 20ft wide line must make a DC 18 Dexterity saving throw. On a failed save, the creature takes 20 (3D8+8) bludgeoning damage and are knocked prone. On a succesful save the creature takes half damage and is merely moved aside. 

Bouncing snatch (Costs 1 Actions). The Bandersnatch jumps towards a target locations within 60ft. All creatures in the target area must make a DC 18 Dexterity saving throw. On a failed save the creature takes 20 (3D8+8) bludgeoning damage and is restrained under the Bandersnatch.