Carrion monster

Amphibious. The Carrion monster can breathe air and water

Amorphous. The Carrion monster can squeeze through a gap as narrow as 1 feet without an action and without being slowed down, regardless of its size.

Amalgamation of flesh. The Carrion monster is a huge creature when above 120hp, a large creature when above 60hp and a medium creature when it is at or under 60hp

Carrion feeder. If the Carrion monster reduces a creature to 0 hp with a bite attack, it rips a part of their flesh off and absorbs it within itself, regaining 20 hp. 

Ravenous. As a bonus action, the Carrion monster can pull a creature it has restrained up to 15 ft. towards it. The creature must succeed a DC 16 Strength saving throw or be pulled towards the monster. On a success, the creature doesn't move.

Energy storage. If the carrion monster takes lightning damage, all of its abilities that require a recharge are charged instantly.

Multiattack. The Carrion monster makes two tentacle attacks. The Carrion monster can use one additionnal bite attack if it is able to.

Tentacles (any form). Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit:  10 (1d10 + 4) bludgeoning damage. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the monster can't use its tentacle on another target. The Carrion monster has 3 prehensile tentacles if it is huge, 2 if it is large and 1 if it is medium.

Bite (any form). Melee Weapon Attack:  +8 to hit, reach 5 ft., one target the carrion monster has restrained. Hit: 15 (2d10 + 4) piercing damage.

Harpoon rush (only when Huge). Each creature within a 30 ft. cone must succeed on a DC 16 Dexterity saving throw or take 21 (6d6) piercing damage and be pulled 15 ft towards the Carrion monster. On a success, a creature only takes half damage and isn't pulled.

Blade charge (only when Large). The Carrion monster extends blades from its body and charges 15 ft. forwards (this movement does not cause opportunity attacks). Each creature within a 10 ft. wide, 15 ft. long line in the direction of the movement must make a DC 16 Strength saving throw or take 21 (6d6) slashing damage and be knocked prone. A creature that succeeds on the saving throw takes half damage and isn't knocked prone.

Impact web (only when Medium). Ranged Weapon Attack: +8 to hit, reach 40 ft., one target. Hit:  11 (2d6 + 4) bludgeoning damage. If the target is a creature, it is restrained by webbing and must make a DC 16 Strength saving throw or be knocked prone. As an action, the restrained target can make a DC 16 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 10; hp 5; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).

Defensive maneuver (Recharge 4-6). The Carrion monster can use one of the following effects, depending on its size :

Defensive shell (Huge form only, in reaction to taking damage). The Carrion monster creates a hard shell around the entirety of its body, reducing all damage the monster recieves by half. The shell falls off at the end of the current turn.

Defensive spikes (Large form only, in reaction to a melee attack). Sharp spikes erupt from the Carrion monster's body, dealing 10 (4d4) piercing damage to the attacker if it is within 5 feet of it. Until the start of the monster's next turn, any creature that does a melee attack while within 5 feet of the monster takes 5 (2d4) piercing damage.

Invisible escape (Medium form only, in reaction to being attacked). The Carrion monster turns invisible right before the attack, and can move up to 20 ft. after the attack. The Carrion monster's invisibility ends at the start of its next turn.