Hulking Shadow

Whenever the shadow is subjected to Acid damage, it takes no damage and instead regains a number of hit points equal to the Acid damage dealt.

Whenever the shadow starts its turn with 60 hit points or fewer, roll a d6. On a 6, the shadow goes berserk. On each of its turns while berserk, the shadow attacks the nearest creature it can see. If no creature is near enough to move to and attack, the shadow attacks an object, with preference for an object smaller than itself. Once the shadow goes berserk, it continues to do so until it is destroyed or regains all its hit points.

The shadow is immune to any spell or effect that would alter its form.

The shadow has advantage on saving throws against spells and other magical effects.

The shadow's weapon attacks are magical.

The shadow makes two slam attacks.

Melee Weapon Attack +8 to hit, reach 5 ft., one target. 16 (2d10 + 5) Bludgeoning damage. If the target is a creature, it must succeed on a DC 15 Constitution saving throw or have its hit point maximum reduced by an amount equal to the damage taken. The target dies if this attack reduces its hit point maximum to 0. The reduction lasts until removed by the Greater Restoration spell or other magic.

Until the end of its next turn, the shadow magically gains a +2 bonus to its AC, has advantage on Dexterity saving throws, and can use its slam attack as a bonus action.