Black knight

Regeneration. The black knight regains 20 hit points at the start of its turn. If the black knight takes acid or fire damage, this trait doesn't function at the start of the black knigth's next turn. The black knight dies only if it starts its turn with 0 hit points and doesn't regenerate.

Loathsome Limbs. Whenever the black knight takes at least 15 slashing damage at one time, roll a d20 to determine what else happens to it:
1-10: Nothing else happens.
11-14: One leg is severed from the black knight if it has any legs left.
15-18: One arm is severed from the black knight if it has any arms left.
19-20: The black knight is decapitated, but the  black knight dies only if it can't regenerate. If it dies, so does the severed head.

If the black knight finishes a short or long rest without reattaching a severed limb or head, the part regrows. At that point, the severed part dies. Until then, a severed part acts on the black knight's initiative and has its own action and movement. A severed part has AC 16, 20 hit points, and the black knight's Regeneration trait.
A severed leg is unable to attack and has a speed of 5 feet.
A severed arm has a speed of 5 feet and can make one great sword attack on its turn, with disadvantage on the attack roll unless the black knight can see the arm and its target. 
The severed head has a speed of 0 feet and can still talk. 
The black knight's speed is halved if it's missing a leg. If it loses both legs, it falls prone. If it has both arms, it can crawl. With only one arm, it can still crawl, but its speed is halved. With no arms or legs, its speed is 0, and it can't benefit from bonuses to speed.

Multi attack. The black knight makes three great sword attack

Great sword attack. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 11 (2d6 + 4) slashing damage. 

Parry. The black knight adds 3 to its AC against one melee attack that would hit it. To do so, the black knight must see the attacker and be wielding a melee weapon.