Corpse Monstrosity

Aversion of Fire. If the monstrosity takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn. 

Corpse Consume. If the monstrosity kills a creature, it can use an action to eat the corpse healing 10 hit points and increasing in size. Every 5 corpses consumed permanently increases all stats by 1 and every 10 corpses increases the size category to the next size up.

Immutable Form. The monstrosity is immune to any spell or effect that would alter its form.
Lightning Absorption. Whenever the monstrosity is subjected to lightning damage, it takes no damage and instead regains a number of hit points equal to the lightning damage dealt.

Magic Resistance. The monstrosity has advantage on saving throws against spells and other magical effects. 
Magic Weapons. The monstrosity's weapon attacks are magical.

Multiattack. The Corpse Monstrosity makes two attacks.

Slam. Melee Weapon Attack: +12 to hit, reach 10 ft., 1 target. Hit: 22 (3d10 + 7) bludgeoning damage. 

Corpse Fling. RangedWeapon Attack: +12 to hit, range 30/60 ft., 1 target. Hit: 15 (2d8 + 7) bludgeoning damage.

Bile Spray. (Recharge 5-6). The corpse monstrosity exhales a line of poison bile that is 20 feet long and 5 feet wide. Each creature in that line must make a DC 16 Constitution saving throw, taking 32 (8d8) poison damage and become poisoned on a failed save, or half as much damage on a successful one.

The corpse monstrosity can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creatures turn. The corpse monstrosity regains spent legendary actions at the start of its turn.

Slam Attack. The corpse monstrosity makes a slam attack.

Bile Spray Attack (Costs 2 Actions). The Corpse Monstrosity immediately makes a bile spray attack even if it had not recharged after a previous use.