Cursed berserker

Magic resistance  The berserker has advantage on saving throws from spells and other magical effects. 

Adaptive form: As a bonus action the berserker can grow new limbs or modify existing ones to suit its needs, it can grow wings, fins, gills or other adaptations to navigate its environment, additionally if its form would be altered by magic it such as polymorph, it may use its action to end the effect on itself. 

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

Multiattack The berserker makes 3 attacks, one with its grasping hand and two with either its blade arm or its jet fist interchangeably. 

Blade arm: +13 to hit, 20 ft., 1target. Hit:  (2d6+7) slashing damage and (1d8) force damage 

Grasping hand +13 to hit, 5 ft, hit: (4d8+7) bludgeoning damage and 1d8 force damage, on a hit a large or smaller creature is grappled. The berserker has advantage to hit a grappled creature. if the berserker uses this attack on an already grappled creature it takes (6d8)+7 bludgeoning damage and (3d8) force damage

Jet fist +13 to hit, 5 ft, (2d8)+7 bludgeoning damage and (2d8) force damage. If the berserker moves 10 or more feet before making this attack the target takes an additional (2d8) force damage and must make a DC 18 strength saving throw or be knocked prone. 

Adaptive form: As a bonus action the berserker can grow new limbs or modify existing ones to suit its needs, it can grow wings, fins, gills or other adaptations to navigate its environment